LIFE HISTORY CHARACTERISTICS AND HABITAT QUALITY OF FLAMMULATED OWLS (OTUS FLAMMEOLUS) IN COLORADO By BRIAN DWIGHT LINKHART B.S., Colorado State University, 1981 M.S., Colorado State University, 1984 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Environmental, Population, and Organismic Biology 2001
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LIFE HISTORY CHARACTERISTICS AND HABITAT QUALITY OF
FLAMMULATED OWLS (OTUS FLAMMEOLUS) IN COLORADO
By
BRIAN DWIGHT LINKHART
B.S., Colorado State University, 1981
M.S., Colorado State University, 1984
A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillment
of the requirement for the degree of
Doctor of Philosophy
Department of Environmental, Population, and Organismic Biology
2001
ii
This thesis entitled:
Life History Characteristics and Habitat Quality of Flammulated Owls (Otus flammeolus) in Colorado
written by Brian Dwight Linkhart
has been approved for the Department of Environmental, Population, and Organismic Biology
________________________________________ Dr. Carl Bock, committee chair
_________________________________________ Dr. Dave Chiszar
_________________________________________ Dr. Sharon Collinge
_________________________________________
Dr. Alexander Cruz
_________________________________________ Dr. Yan Linhart
Date ____________________________
The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly
work in the above mentioned discipline.
iii
Linkhart, Brian Dwight (Ph.D., Environmental, Population, and Organismic Biology) Life history characteristics and habitat quality of Flammulated Owls (Otus
flammeolus) in Colorado
Thesis directed by Dr. Carl E. Bock
Abstract. – I determined the life history characteristics and the components of habitat
quality in a Colorado population of Flammulated Owls (Otus flammeolus) in a 19-yr
study. The owl is a small, monogamous Neotropical migrant that nests in mature
conifer forests in western North America, and is considered sensitive by the USDA
Forest Service. Conservation planning is limited by lack of data regarding the owl’s
population dynamics and habitat requirements. I assessed population dynamics based
on density, territory fidelity, dispersal, survival, and reproduction of the owls. Most
owl territories were constant in time and space despite turnover of individuals.
Density of breeding pairs showed little annual variation. Up to 70% of territories
were occupied annually by bachelor males, suggesting that females have lower
survival. Compared to other North American owls, Flammulated Owls have a small
and unvarying clutch size, high nesting success, and a long breeding lifespan,
indicating they have a life history similar to larger owls.
Territory fidelity was male-biased, as it is with most birds, and pairs rarely
divorced. Most breeding dispersals were by females that moved one or two territories
away from their original territories. However, females whose nests failed the previous
year had lower return rates to the study area than females whose previous nests were
iv
successful. Dispersal distance may be bimodal with females dispersing longer distances
after nesting failure and shorter distances after successful nests. Females dispersed to
territories where total productivity during the study was higher than on original territories,
suggesting they assessed territory quality before dispersing.
Characteristics of high-quality breeding habitats were determined by
correlating long-term demographic parameters of owls with habitat characteristics on
their territories. Territories differed significantly in total years they were occupied by
breeding pairs and in total productivity. Availability of cavity-trees for nesting
determined where owls established territories, while forest type and structure
determined whether a territory was more often occupied by breeding pairs or bachelor
males. High-quality breeding habitat for Flammulated Owls was characterized as
mature, open stands of ponderosa pine (Pinus ponderosa) mixed with Douglas-fir
(Pseudotsuga menziesii) with sufficient cavity-trees for nesting.
v
Acknowledgements
I am grateful for all of the people who provided support for this project. First,
I thank my major advisor, Dr. Carl Bock, for his guidance, advice, and support
throughout this project. I also extend special thanks to my committee members, Dr.
Dave Chiszar, Dr. Sharon Collinge, Dr. Alexander Cruz, Dr. Yan Linhart, and Dr.
Carol Wessman.
Had it not been for the U.S. Forest Service, this project could never have
started or continued over the past 19 years. The Rocky Mountain Research Station
always has been the primary source of funding and logistical support. The Manitou
Experimental Forest proved to be a perfect location for this long-term study, offering
as natural and unmodified montane forest as might be found anywhere in the region.
My discoveries and experiences on this forest always will be part of my memories.
Several employees of the U.S. Forest Service have had vital roles in this
study. I am particularly grateful to Dr. Richard T. Reynolds, who has had a marked
effect on my career. Dr. Reynolds first offered me a job as a summer technician
assisting his raptor research projects throughout Colorado when I was an
undergraduate at Colorado State University, and he sparked my interests in research
and taught me how to become a good field biologist. Over the years his supervision,
support, encouragement, and friendship have been essential to my success. I am
indebted to his efforts.
I also thank Carl Edminster and Dr. Wayne Shepperd, who have administered
the Manitou Experimental Forest over the years, and William Knott, Ross Watkins,
vi
and Steve Tapia, who as forest caretakers always were eager to help. I especially
thank Steve Tapia, who always has gone out of his way to assist with arranging and
procuring necessary equipment and materials, helping in the field, and locating
potential volunteers. He also has been a good friend. I thank Mike Morrison and
Frank Romero, who were instrumental in providing digitized data for use in
Geographic Information Systems (GIS), and Mark Roper, who assisted with GPS
equipment. Without the friendship and advice of Suzanne Joy, who dispensed
invaluable advice on GIS matters day or night, I might still be hopelessly punching
the keyboard. Finally, Rudy King was especially helpful in assisting me with
statistical analyses and his advice was greatly appreciated.
I am very grateful to several people in the Geography Department at the
University of Colorado who provided technical skill, advice, and computer equipment
for my GIS analyses including Dr. Barbara Buttenfield, Sara Farenkopf, Jim Robb,
Chris Hanson, and Tom Dickinson. I am particularly grateful to Dr. Robin Reich,
Department of Forest Sciences at Colorado State University, for providing several
months of assistance and instruction with vegetation modeling.
A project of this duration necessitates a large number of field workers and I
am very grateful to the many individuals who offered their assistance in data
collection. Especially notable were several individuals who each volunteered over
four hundred hours, including Buzz Morrison, Scott Hiller, Patrick Leavey, and
Nicholas Tucey. The commitment of these individuals, and the extent to which they
benefited my study, cannot be understated. Also assisting with field work were Josie
Bamford, Holly Barnard, Christina Bauman, Cloud Ridge Naturalists, Sam Berry,
vii
Rob Bringuel, Michelle and Tim Connelly, Peg Coontz, Dave and Laura (Scott)
Denton, Peter Gaede, Jennifer Gass, Matt Harper, Emily Holt, Stephanie Howard,
Sam Johnson, Littleton High School students, Steve Mata, Terry and Amelia
McGlynn, William Merkle, Monica Mohr, Monica Molachy, Mark and John
Morgenstern, Nickolas Neitzel, Charlie Nemkevich, Andrew Orahoske, Summer
Orniz, Amy Ottaway, Amy Pabski, Mark Platten, Jenna Sanchez, David Stahl, Sue
Stern, Greg Styduhar, and Steve Tapia.
In addition to the Rocky Mountain Research Station, financial support was
provided by Littleton Public Schools, the University of Colorado, the Department of
Environmental, Population, and Organismic Biology, the Colorado Mountain Club
Foundation, Colorado Natural History Small Grants, and the Cooper Ornithological
Society.
The support I have received from my friends and family have been profoundly
important. I thank Audrey and Jim Benedict, and Tim and Michelle Connelly, whose
friendship and encouragement have been sources of strength for over two decades. I
particularly thank my mom and dad, whose love of nature was instilled in me at a
very young age. It is to them that I owe my curiosity of life, and for that I will always
be deeply appreciative. Finally, I am especially grateful for my wife, Marlene, who
provided tremendous love and support through all years of this project when I was
spending innumerable hours at night running over mountains in search of singing
owls and hooting at the moon. She truly has been the wind in my sails and I owe the
completion of this effort to her.
viii
Table of Contents
CHAPTER
I. BACKGROUND AND OUTLINE OF DISSERTATION TOPICS
Introduction………………………………………………………… 1
Outline of dissertation topics………………………………………. 4
Literature Cited ……………………………………………………. 7
II. DEMOGRAPHY OF FLAMMULATED OWLS
Abstract …………………………………………………………… 11
Introduction ………………………………………………………. 13
Methods …………………………………………………………… 14
Results ……………………………………………………………. 21
Discussion ………………………………………………………… 33
Literature Cited …………………………………………………… 38
III. LIFETIME REPRODUCTION OF FLAMMULATED OWLS
Abstract …………………………………………………………… 43
Introduction ……………………………………………………….. 44
Methods …………………………………………………………… 46
Results …………………………………………………………….. 49
Discussion …………………………………………………………. 61
Literature Cited ……………………………………………………. 67
ix
IV. MATE AND SITE FIDELITY AND DISPERSAL IN
FLAMMULATED OWLS
Abstract …………………………………………………………… 70
Introduction ……………………………………………………….. 72
Methods …………………………………………………………… 74
Results …………………………………………………………….. 81
Discussion …………………………………………………………. 89
Literature Cited …………………………………………………… 98
V. DETERMINING HABITAT QUALITY FROM LONGTERM
DEMOGRAPHICS IN FLAMMULATED OWLS
Abstract …………………………………………………………… 105
Introduction ……………………………………………………….. 108
Methods …………………………………………………………… 112
Results …………………………………………………………….. 128
Discussion ………………………………………………………… 159
Literature Cited ……………………………………………………. 170
VI. CONCLUSIONS ………………………………………………….. 180
Literature Cited ……………………………………………………. 186
VII. LITERATURE CITED ……………………………………………. 189
x
List of Tables
Table 1 Mate improvement hypothesis; comparison of lifetime
production of owlets by original males on territories from
which females dispersed to lifetime production of owlets
by new males on territories where females dispersed.
………. 88
Table 2 Territory improvement hypothesis; comparison of total
owlets produced over 19 yr on territories where females
nested before dispersal (“original territory”) to total
owlets produced on territories where females dispersed
(“new territory”).
………. 90
Table 3 Disperser-enhancement hypothesis; comparison of mean
brood size for females before they dispersed to mean
brood size on territories after they dispersed.
………. 91
Table 4 Forest overstory and understory variables used in
correlations with demographic variables.
………. 120
Table 5 Demography on owl territories from 1981-1999. ………. 131
Table 6 Comparisons of owlets over 19 yr on territories where ……….. 138
xi
females nested before dispersal (“original territory”) to
owlets produced on territories to which those same
females dispersed (“new territory”).
Table 7 Area and percentage of forest types in owl territories,
non-territory habitat, and over the entire study area.
………. 139
Table 8 Comparison of forest structure variables among forest
types.
………. 141
Table 9 Correlations among demographic variables and forest
types within owl territories.
………. 147
Table 10 Correlations among demographic and forest structure
variables within owl territories.
………. 148
Table 11 Comparison of forest structure variables in territory vs
non-territory habitat.
………. 151
Table 12 Comparison of forest structure variables among non-
territory and three classes of territory habitat.
………. 153
Table 13 Density of cavity trees among owl territories and in non- ………. 156
xii
territory habitat.
Table 14 Comparison of relative arthropod abundance between a
high-productivity territory (A4) and a low-productivity
territory (A18) during 1998-1999.
………. 158
xiii
List of Figures
Figure 1 Location of owl territories on the Manitou
Experimental Forest study area, 1981-1999.
………. 16
Figure 2 Number of territories occupied annually by bachelor
(unpaired) males and breeding pairs, 1981-1999.
………. 23
Figure 3 Number of nesting attempts and successful nests per
yr on 14 territories, 1981-1999.
………. 25
Figure 4 Annual mean number of eggs per clutch, number of
owlets per brood, and number of owlets per successful
brood, 1981-1999.
……….. 26
Figure 5 Total eggs and fledglings produced in all territories
annually from 1981-1999.
………. 28
Figure 6 Total years of return to the study area by male and
female owls.
………. 31
Figure 7 Estimated survival curves for male and female owls. ………. 32
xiv
Figure 8 Lifetime number of nesting attempts by female and
male owls.
………. 51
Figure 9 Lifetime number of successful nests by female and
male owls.
………. 52
Figure 10 Lifetime percentage of successful nests by female and
male owls.
………. 53
Figure 11 Lifetime production of eggs by female and male owls. ………. 55
Figure 12 Lifetime production of fledglings by female and male
owls.
………. 56
Figure 13 Relationship between total breeding years and total
fledglings for female and male owls.
………. 58
Figure 14 Relationship between total eggs and fledglings for
female and male owls.
………. 59
Figure 15 Percent of total fledglings produced by varying
percentages of female and male owls.
………. 60
xv
Figure 16 Lifetime number of new mates for female and male
owls.
………. 62
Figure 17 Location of owl territories on the Manitou
Experimental Forest study area, 1981-1999.
………. 82
Figure 18 Location of owl territories on the Manitou
Experimental Forest study area, 1981-1999.
………. 115
Figure 19 Contribution of individual territories to total owlets
produced by combined territories from 1981-1999.
………. 136
Figure 20 Distribution of owl territories, cavity trees, and forest
types on the Manitou Experimental Forest study area.
………. 140
Figure 21 Correlations between demographic variables and
forest types across owl territories.
………. 146
CHAPTER I
BACKGROUND AND OUTLINE OF DISSERTATION TOPICS
Life histories of avian species have evolved to maximize lifetime reproductive
output in particular environments (Stearns 1992). Life histories, which consist of
coevolved characteristics including reproductive rate, natal and breeding dispersal,
and breeding life span, vary markedly across avian species (Moreau 1944, Lack 1954,
Ricklefs 2000). Despite variance in life histories, studies of demography have
revealed several internally-consistent patterns such as correlation between body mass,
fecundity and longevity. Large species, including many raptors, are generally long-
lived and have low fecundity while small species are typically short-lived and have
high fecundity (Newton 1979, Johnsgard 1988, Gill 1995). Large, long-lived species
also generally have low rates of annual turnover with more overlap among
generations compared to small, short-lived species (Newton 1998, Ricklefs 2000).
Investigations of life histories are important in population ecology for several
reasons. First, life history characteristics such as breeding density, reproduction, and
survival determine population dynamics. Studies of these characteristics reveal the
relative stability of populations in time and space (Grant 1986, Woolfendon and
Fitzpatrick 1984), and provide insight in determining how populations are affected by
ecological factors such as habitat quality (Newton and Marquiss 1982, Forsman et al.
1996), prey abundance (Nol and Smith 1987, Korpimaki 1988), nest predation
2
(Martin 1988, Bosque and Bosque 1995), and extreme weather (Grant 1986, Owen
and Black 1989). Life history characteristics are also important in developing avian
conservation strategies because species may respond differently to environmental
perturbations based on patterns in their reproduction and survival (Newton 1995,
Forsman et al. 1996).
Second, life history investigations that focus on lifetime reproductive success
(LRS), the total offspring raised by individuals over their lifetimes, are useful for
determining reproductive strategies of species and variance in productivity among
individuals. For example, LRS data show that Kingfishers (Alcedo atthis) exhibit a
limited but highly productive breeding life, breeding annually for no more than 4 yr,
and producing more than 20 young per yr (Bunzel and Druke 1989). In contrast, LRS
data show that Barnacle Geese (Branta leucopsis) have a long but relatively
unproductive breeding life. These waterfowl initiate breeding at nearly 7 yr, produce
fewer than 5 young per yr, and total number of offspring rarely exceeds 10 in a
lifetime that may be greater than 20 yr (Owen and Black 1989). In both species, less
than one-third of the breeding population produces 50% of all offspring (Bunzel and
Druke 1989, Owen and Black 1989). LRS also provides one of the best estimates of
individual fitness, and may allow identification of individual attributes and
environmental correlates that contribute most importantly to fitness (Newton 1989).
Third, studies of mate and site fidelity (e.g., Greenwood 1980, Gavin and
Bollinger 1988), and natal and adult dispersal (e.g., Greenwood and Harvey 1982,
Forero et al. 1999), are important because movement behaviors strongly influence
population structure and gene flow (Johnson and Gaines 1990). Moreover,
3
understanding the ecological correlates of dispersal is useful because factors such as
duration, and breeding dispersal) on territories, and identify variables that
distinguished among territories; and (2) identify components of habitat quality by
correlating habitat variables with demographic performance on territories. I
evaluated habitat quality based on two questions: (a) Across territories, was
demographic performance associated with forest type and structure (e.g., tree density,
basal area, and crown volume)? (b) Does forest structure differ among territories and
between territory and non-territory (i.e., unoccupied) habitat? I predicted that
reproductive success could be positively associated with area in ponderosa
pine/Douglas-fir, a forest type and structure with which Flammulated Owls have been
associated in other studies (McCallum 1994). I also evaluated three possible limiting
factors associated with the owl’s habitat relationships, and predicted that highest-
quality territories were characterized by highest densities of cavity trees, lowest rates
of nest predation, and greatest prey abundance.
7
LITERATURE CITED
Bosque, C., and M. T. Bosque. 1995. Nest predation as a selective factor in the evolution of developmental rates in altricial birds. Amer. Natural. 145:234-260.
Bull, E. L., and E. G. Anderson. 1978. Notes on Flammulated Owls in northeastern
Oregon. Murrelet 59:26-27. Bunzel, M. and J. Druke. 1989. Kingfisher. Pages 107-117 in Lifetime reproduction
in birds (I. Newton, Ed.). Academic Press, San Diego, California. Coulson, J. C. 1966. The influence of the pair-bond and age on the breeding biology
of the kittiwake gull Rissa tridactyla. J. Anim. Ecol. 35:269-279. Ens, B. J., M Kersten, A Brenninkmeijer, and J. B. Hulscher. 1992. Territory
quality, parental effort and reproductive success of Oystercatchers (Haimatopus ostralegus). J. Anim. Ecol. 61:703-715.
Forero, M. G., J. A. Donozar, J. Blas, and F. Hiraldo. 1999. Causes and
consequences of territory change and breeding dispersal distance in the Black Kite. Ecol. 80: 1298-1310.
Forsman, E.D., S. Destefano, M. G. Raphael, and R. J. Gutierrez. 1996.
Demography of the Northern Spotted Owl. Studies in Avian Biol. No. 17, Cooper Ornithol. Soc.
Franzreb, K. E., and R. D. Ohmart. 1978. The effects of timber harvesting on
breeding birds in a mixed-coniferous forest. Condor 80:431-441. Gavin, T. A., and E. K. Bollinger. 1988. Reproductive correlates of breeding-site
fidelity in Bobolinks (Dolichonyx oryzivorus). Ecol. 69:96-103. Gill, F. B. 1995. Ornithology. W. H. Freeman and Co., New York. 2nd Ed. 766 pp. Goodburn, S. F. 1991. Territory quality or bird quality? Factors determining
breeding success in the Magpie Pica pica. Ibis 133:85-90. Grant, P. R. 1986. Ecology and evolution of Darwin’s Finches. Princeton Univ.
Press, Princeton, New Jersey. Greenwood, P. J. 1980. Mating systems, philopatry, and dispersal in birds and
mammals. Anim. Behav. 28:1140-1162. Greenwood, P. J., and P. H. Harvey. 1982. The natal and breeding dispersal of birds.
Ann. Rev. of Ecol. and Syst. 13:1-21.
8
Haas, C. 1998. Effects of prior nesting success on site fidelity and breeding
dispersal: an experimental approach. Auk 115:929-936. James, F. C., and N. O. Warner. 1982. Relationships between temperate forest bird
communities and vegetation structure. Ecol. 63:159-171. Johnson, M. L., and M. S. Gaines. 1990. Evolution of dispersal: theoretical models
and empirical test using birds and mammals. Ann. Rev. Ecol. Syst. 21:449-480.
Johnsgard, P. A. 1988. North American owls: Biology and natural history.
Smithsonian Institution Press, Washington, D.C. Korpimaki, E. 1988. Effects of territory quality on occupancy, breeding performance
and breding dispersal in Tengmalm’s Owl. J. Anim. Ecol. 57:97-108. Lack, D. 1954. The natural regulation of animal numbers. Oxford, University Press. Linkhart, B. D. and R. T. Reynolds. 1997. Territories of Flammulated Owls: Is
occupancy a measure of habitat quality? Pp. 250-254 in Biology and conservation of owls of the northern hemisphere (J. R. Duncan, D. H. Johnson, and T. H. Nichols, Eds.). USDA Forest Serv. Gen Tech. Rep. NC-190.
Linkhart, B. D., R. T. Reynolds, and R. A. Ryder. 1998. Home range and habitat of
breeding Flammulated Owls in Colorado. Wilson Bull.. 110:342-351. Marshall, J. T., Jr. 1957. Birds of pine-oak woodland in southern Arizona and
adjacent Mexico. Pacif. Coast Avif. 32:1-125. Marshall, J. T., Jr. 1988. Birds lost from a giant sequoia forest during fifty years.
Condor 90:359-372. Martin, T. E. 1988. Processes organizing open-nesting bird assemblages:
competition or nest predation. Evol. Ecol. 2:37-50. Martin, T. E. 1992. Breeding productivity considerations: what are the appropriate
habitat features for management? Pages 455-473 in Ecology and conservation of Neoptropical migrant landbirds (J. M. Hagan III and D. W. Johnston, eds.). Smithsonian Institution Press, Washington, D.C.
Maurer, B. A. 1986. Predicting habitat quality for grassland birds using density-
habitat correlation. J. Wildl. Manage. 50:556-566.
9
McCallum, A. 1994. Flammulated Owl (Otus flammeolus). In The birds of North America, No. 93 (A. Poole and F. Gill, eds.). Academy of Natural Sciences of Philadelphia; American Ornithologists’ Union, Washington, D.C.
Moreau, R. E. 1944. Clutch size: a comparative study, with references to African
birds. Ibis 86:286-347. Newton, I. 1979. Population ecology of raptors. T & A D Poyser Ltd., London. 399
pp. Newton, I. 1989. Lifetime reproduction in birds. Academic Press, San Diego,
California. Newton, I. 1995. The contribution of recent research on birds to ecological
understanding. J. Anim. Ecol. 64:675-696. Newton, I. 1998. Population limitation in birds. Academic Press, N.Y. 597 pp. Newton, I., and M. Marquiss. 1982. Fidelity to breeding area and mate in
Sparrowhawks Accipiter nisus. J. Anim. Ecol. 51:327-341. Nol, E., and J. N. M. Smith. 1987. Effects of age and breeding experience on
seasonal reproductive success in the Song Sparrow. J. Anim. Ecol. 56:301-313.
Owen, M., and J. M. Black. 1989. Barnacle Goose. Pages 349-362 in Lifetime
reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, CA. Paradis, E., S. R. Baillie, W. J. Sutherland, and R. D. Gregory. 1998. Patterns of
natal and breeding dispersal in birds. J. Anim. Ecol. 67:518-536. Phillips, A. R., J. T. Marshall, and G. Monson. 1964. The birds of Arizona. Univ.
Ariz. Press, Tucson. 212 pp. Ricklefs, R. E. 2000. Density dependence, evolutionary optimization, and
diversification of avian life histories. Condor 102:9-22. Stearns, S. C. 1992. The evolution of life histories. Oxford Univ. Press, New York.
249 pp. Van Horne, B. 1983. Density as a misleading indicator of habitat quality. J. Wildl.
Manage. 47:893-901. Van Horne, B., G. S. Olson, R. L. Schooley, J. G. Corn, and K. P. Burnham. 1997.
Effects of drought and prolonged winter on Townsend’s Ground Squirrel demography in shrubsteppe habitats. Ecol. Monogr. 67:295-315.
10
Verner, J. 1994. Current management situation: Flammulated Owls. Pages 10-13 in
Flammulated, Boreal, and Great Gray Owls in the United States: a technical conservation assessment (G. D. Hayward and J. Verner, eds.). USDA Forest Serv. Gen. Tech. Rep. RM-253.
Wenny, D. G., R. L. Claswon, J. Faaborg, and S. L. Sheriff. 1993. Population
density, habitat selection, and minimum area requirements of three forest-interior Warblers in central Missouri. Condor 95:968-979.
Whitmore, R. C. 1977. Habitat partitioning in a community of passerine birds.
Wilson Bull. 89:253-265. Woolfenden, G. E., and J. W. Fitzpatrick. 1984. The Florida Scrub Jay:
demography of a cooperative-breeding bird. Monogr. Pop. Biol. 20, Princeton Univ. Press, Princeton, NJ.
van Woudenberg, A. M. 1992. Integrated management of Flammulated Owl
breeding habitat and timber harvest in British Columbia. Masters thesis, Univ. British Columbia, Vancouver.
11
CHAPTER II
DEMOGRAPHY OF FLAMMULATED OWLS IN COLORADO
Abstract. - I investigated the demography of Flammulated Owls (Otus flammeolus) in
Colorado from 1981-1999. Fourteen territories occurred on the 511 ha study area
during the 19 yr study, most of which were constant in time and space despite
periodic turnover of individuals on territories. Mean (+ SE) annual density was 0.9 +
g), much larger than Flammulated Owls (50-66 g; Reynolds and Linkhart 1987), have
a smaller clutch size (-x = 2.4 eggs/clutch for both species; Murray 1976, Johnson
1978). Second, at least 75% of nests were successful in 16 of 19 years for an overall
nesting success rate of 82% over the study. Among North American strigiforms, only
Spotted Owls (S. occidentalis) have a higher nesting success (85%; Forsman et al.
1984, Johnsgard 1988). Third, I never observed replacement clutches or multiple
broods in Flammulated Owls even when nests failed early in the breeding season.
Although multiple broods are relatively uncommon among raptors (but see Marti
1997), several small and some medium-sized species in temperate regions may lay
replacement clutches if their first clutch is lost in early incubation (Newton 1979,
Johnsgard 1988). Finally, while longevity is not known for most North American
34
strigiforms, male Flammulated Owls have greater maximum longevity (12+ yr) than
other small (< 150 g) owls and a longevity comparable to many larger owls (Glutz
and Bauer 1980, Clapp et al. 1983, Klimkiewicz and Futcher 1989, Klimkiewicz,
pers. comm). Coupled with low turnover rates on territories (Chapter 5), these data
suggest that male Flammulated Owls have relatively high annual survival, despite
their being the most migratory of North American strigiforms (Johnsgard 1988).
Breeding Densities
Density of territories occupied by breeding pairs varied little over the 19 yr
study; breeding pairs occupied either 4 or 5 territories in 79% (15 of 19) of yr. The
relatively constant annual breeding density suggests that the owls in the study area
occupied a relatively stable environment (sensu Pianka 1970). Other long-term
studies of raptors including strigiforms showed stable breeding populations over time
(e.g., Gargett 1977, Korpimaki 1988).
Underlying the relative constancy in annual density of breeding-pairs was the
fact that many of the same territories were reoccupied nearly every year by breeding
pairs. As a consequence of the continuous occupancy, these territories accounted for
the majority of total productivity over the study. Elsewhere I reported that 5 of 12
territories were occupied by breeding pairs > 8 yr (not necessarily consecutive) and
accounted for nearly 70% of total owlets produced on the study area from 1981-1999,
and 3 territories, occupied by breeding pairs for > 11 yr, accounted for 50% of total
owlets produced (Chapter 5). Productivity on territories was higher on territories
with mature, open forests of ponderosa pine/Douglas-fir forests (Chapter 5),
suggesting that availability of this forest type and structure were factors associated
35
with the relative constancy in density and reproduction of Flammulated Owls on the
study area. Other studies inferred that demographic parameters were most constant in
high-quality breeding habitats (e.g., Probst and Hayes 1987, Korpimaki 1988,
Steenhof et al. 1999).
Patterns in demographic characteristics in birds are often closely related to
food abundance. Several studies have found that annual densities of raptor
populations were strongly correlated with prey density (e.g., Craighead and
Craighead 1956,White and Cade 1971, Korpimaki 1988). Reproductive parameters
such as nesting success and clutch size are closely tied to food abundance in several
species of strigiforms (Johnsgard 1988) and other raptors (Steenhof et al. 1999). The
constancy of Flammulated Owl demographic characteristics during the study suggests
that prey generally was a reliable resource. Indeed, long-term owl productivity was
not associated with prey abundance sampled over 2 yr (Chapter 5), suggesting that
prey abundance was not limiting and was not associated with habitat quality.
Sex Differences In Longevity
Greater estimated longevity for male Flammulated Owls suggests a sex bias in
survival, emigration, or both. Mean total yr of return was significantly greater for
males (3.2 + 0.6 yr) than for females (2.0 + 0.3 yr), and 40% of males returned for > 3
yr to the study area compared to just 18% of females. Annual frequency of return
may give a biased estimate of survival because (1) females had higher rates of
detected breeding dispersal within the study area than males, and (2) females had
lower return rates following nest failure than males (Chapter 4). These data suggest
that some of the females that had not returned may have dispersed from the study area
36
(Chapter 4). Nonetheless, the fact that unpaired males annually occupied 10-70% of
the 14 territories strongly suggests a shortage of females. If differences in return rates
for males and females are related to survival, the underlying reasons for these
differences are uncertain. A possible explanation is that the cost of egg production in
female Flammulated Owls, whose clutches represent more of their mass (55-60%;
approximately 30 g) than most other strigiforms (Johnsgard 1988), coupled with
energetic costs of long-distance migration prior to egg-laying, may result in higher
adult female mortality. Other studies of monogamous birds have reported or
suggested a male-biased sex ratio in adults (e.g., Breitwisch 1989 and sources therein,
Burke and Nol 1998, Gibbs and Faaborg 1990, Payne and Payne 1990).
Factors Affecting Reproductive Success
Nest predation is a primary cause of nesting mortality for many bird species
(e.g., Skutch 1949, Ricklefs 1969), and is believed to be an important factor in the
evolution of life-histories (Slagsvold 1982, Sonerud 1985, Martin 1988, Bosque and
Bosque 1995). While nest predation was responsible for reducing nesting success in
certain years (e.g., 1993 and 1999), the high rate of nesting success (82%) over 19 yr
suggests that nest predation may not be a major factor influencing reproductive
success. Elsewhere I reported that, among territories differing in long-term
productivity, predation rates at artificial nests were not significantly different,
indicating that nest predation was not associated with territory quality (Chapter 5).
Contrary to studies that found that reproductive success was correlated with
breeding experience (e.g., Nol and Smith 1987, Pietiainen 1988), male and female
Flammulated Owls with > 2 yr breeding experience did not initiate incubation earlier
37
or have larger broods than males and females breeding for the first time on the study
area. However, if owls bred elsewhere prior to their first breeding on the study area,
then any differences between inexperienced (first-time) or experienced breeders
would have been diluted. Moreover, since adults at first capture could not be aged, I
could not separate the effects of breeding age from the effects of breeding experience.
Age, rather than experience per se, was known to affect reproductive success in
several birds (e.g., Harvey et al. 1979, Curio 1983, Nol and Smith 1987).
Conservation Implications
High-quality breeding habitat for Flammulated Owls in central Colorado was
characterized as mature, open forests of ponderosa pine/Douglas-fir and owls
preferentially foraged in this forest type (Chapter 5; Linkhart et al. 1998). Elsewhere
in its range, the owl has been generally associated with mature conifer forests (see
McCallum 1994), and these forests have been subjected to extensive tree harvesting
over the past several decades. In fact, tree harvesting caused population declines in
Flammulated Owls in some areas (Marshall 1957, 1988, Phillips et al. 1964, Franzreb
and Ohmart 1978). Based on the fact that the Flammulated Owl appears to have a K-
selected life-history strategy, characterized by low rates of reproduction and high
survival (Pianka 1970), these data suggest that the owl may be vulnerable to long-
term habitat changes. K-selected species typically respond slowly to environmental
perturbations because of their low fecundity and low density (Newton 1998). In order
to understand effects of habitat changes on dynamics and long-term viability of owl
populations, researchers need to undertake comparative demographic studies of owls
across multiple forest management regimes.
38
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Breitwisch, R. 1989. Mortality patterns, sex ratios, and parental investment in
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forest fragmentation on breeding Ovenbirds. Auk 115:96-104. Clapp, R. B., M. K. Klimkiewicz, and A. G. Futcher. 1983. Longevity records of
North American birds: columbidae through paridae. J. Field Ornithol. 54: 123-137.
Coulson, J. C. 1966. The influence of the pair-bond and age on the breeding biology
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Co., Pennsylvania. Curio, E. 1983. Why do young birds reproduce less well? Ibis 400-404. Dhondt, A. A. 1989. Blue Tit. Pages 15-33 in Lifetime reproduction in birds (I.
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Forsman, E. D., E. C. Meslow, and H. M. Wight. 1984. Distribution and biology of
the Spotted Owl in Oregon. Wildl. Monogr. 87:1-64. Forsman, E.D., S. Destefano, M. G. Raphael, and R. J. Gutierrez. 1996.
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Franzreb, K. E., and R. D. Ohmart. 1978. The effects of timber harvesting on
breeding birds in a mixed-coniferous forest. Condor 80:431-441. Gargett, V. 1977. A 13-year population study of the Black Eagles in the Matopos,
Rhodesia, 1964-1976. Ostrich 48: 17-27.
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Gibbs, J. P., and J. Faaborg. 1990. Estimating the viability of Ovenbird and Kentucky Warbler populations in forest fragments. Conserv. Biol. 4:193-196.
Grant, P. R. 1986. Ecology and evolution of Darwin’s Finches. Princeton Univ.
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Mitteleuropas, Vol. 9. Akademische Verlagsgesellschaft, Wiesbaden. Harvey, P. H., P. J. Greenwood, C. M. Perrings, and A. R. Martin. 1979. Breeding
success of great tits Parus major in relation to age of male and female parent. Ibis 121:186-200.
Johnsgard, P. A. 1988. North American owls: Biology and natural history.
Smithsonian Institution Press, Washington, D.C. Klimkiewicz, M. K., and A. G. Futcher. 1989. Longevity records of North American
birds. Supplement I. J. Field Ornithol. 60:469-494. Korpimaki, E. 1988. Effects of territory quality on occupancy, breeding performance
and breding dispersal in Tengmalm’s Owl. J. Anim. Ecol. 57:97-108. Lack, D. 1954. The natural regulation of animal numbers. Oxford, University Press. Lebreton, J. D., K. P. Burnham, J. Clobert, and D. R. Andersen. 1992. Modeling
survival and testing biological hypotheses using marked animals: a unified approach with case studies. Ecol. Monogr. 62:67-118.
Linkhart, B. D., R. T. Reynolds, and R. A. Ryder. 1998. Home range and habitat of
breeding Flammulated Owls in Colorado. Wilson Bull.. 110:342-351. Linkhart, B. D., and R. T. Reynolds. 1994. Peromyscus carcass in the nest of a
Flammulated Owl. J. Raptor Res. 28:43-44. Linkhart, B. D. 1984. Range, activity, and habitat use by nesting Flammulated Owls
in a Colorado ponderosa pine forest. M. S. Thesis, Colorado State Univ., Fort Collins. 45 pp.
Marshall, J. T., Jr. 1939. Territorial behavior of the Flammulated Screech Owl.
Condor 41:71-78. Marshall, J. T., Jr. 1957. Birds of pine-oak woodland in southern Arizona and
adjacent Mexico. Pacif. Coast Avif. 32:1-125. Marshall, J. T., Jr. 1988. Birds lost from a giant sequoia forest during fifty years.
Condor 90:359-372.
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Marti, C. D. 1997. Lifetime reproductive success in Barn Owls near the limit of the
species range. Auk 114:581-592. Martin, T. E. 1988. Processes organizing open-nesting bird assemblages:
competition or nest predation. Evol. Ecol. 2:37-50. Martin, T. E., J. Clobert, and D. R. Anderson. 1995. Return rates in studies of life
history evolution: are biases large? J. Appl. Stat. 22:863-875. McCallum, A. 1994. Flammulated Owl (Otus flammeolus). In The birds of North
America, No. 93 (A. Poole and F. Gill, eds.). Academy of Natural Sciences of Philadelphia; American Ornithologists’ Union, Washington, D.C.
Moreau, R. E. 1944. Clutch size: a comparative study, with references to African
birds. Ibis 86:286-347. Murphy, M. T. 1996. Survivorship, breeding dispersal and mate fidelity in Eastern
Kingbirds. Condor 98:82-92. Murray, G. A. 1976. Geographic variation in the clutch sizes of seven owl species.
Auk 93:602-613. Newton, I. 1979. Population ecology of raptors. T & A D Poyser Ltd., London. 399
pp. Newton, I. 1998. Population limitation in birds. Academic Press, New York. 597
pp. Nol, E., and J. N. M. Smith. 1987. Effects of age and breeding experience on
seasonal reproductive success in the Song Sparrow. J. Anim. Ecol. 56:301-313.
Payne, R. B., and L. L. Payne. 1990. Survival estimates of Indigo Buntings:
comparisons of banding recoveries and local observations. Condor 92:938:946.
Perrins, C. M., and T. A. Geer. 1980. The effect of Sparrowhawks on Tit
Populations. Ardea 68:133-142. Phillips, A. R., J. T. Marshall, and G. Monson. 1964. The birds of Arizona. Univ.
Ariz. Press, Tucson. 212 pp. Pianka, E. R. 1970. On r- and K-selection. Amer. Natural. 104:592-597.
41
Pietiainen, H. 1988. Breeding season quality, age, and the effect of experience on the reproductive success of the Ural Owl (Strix uralensis). Auk 105:316-324.
Probst, J. R., and J. P. Hayes. 1987. Pairing success of Kirtland’s Warblers in
marginal vs. suitable habitat. Auk 104:234-241. Reynolds, R. T., and B. D. Linkhart. 1984. Methods and materials for capturing and
monitoring Flammulated Owls. Great Basin Natural. 44:49-51. Reynolds, R. T., B. D. Linkhart, and J. Jeanson. 1985. Characteristics of snags and
trees containing cavities in a Colorado conifer forest. USDA Forest Serv. Res. Note RM-455. 6 pp.
Reynolds, R. T., and B. D. Linkhart. 1987. The nesting biology of Flammulated
Owls in Colorado. Pages 239-248 in Biology and conservation of northern forest owls: symposium proceedings (R. W. Nero, R. J. Clark, R. J. Knapton, R. H. Hamre, eds.). USDA Forest Serv. Gen. Tech.. Rep. RM-142.
Ricklefs, R. E. 1969. An analysis of nesting mortality in birds. Smithson. Contrib.
Zool. 9:1-48. Ricklefs, R. E. 2000. Density dependence, evolutionary optimization, and
diversification of avian life histories. Condor 102:9-22. SAS Institute. 1996. SAS user’s guide: Statistics, version 6.12. SAS Institute, Inc.,
Cary, North Carolina. Skutch, A. F. 1949. Do tropical birds rear as many young as they can nourish? Ibis
91:430-455. Slagsvold, T. 1982. Clutch size variation in passerine birds: the nest predation
hypothesis. Oecologia 54:159-169. Sonerud, G. A. 1985. Risk of nest predation in three species of hole nesting owls:
influence on choice of nesting habitat and incubation behaviour. Ornis Scand. 16:261-269.
Steenhof, K., M. N. Kochert, L. B. Carpenter, and R. N. Lehman. 1999. Long-term
Prairie Falcon population changes in relation to prey abundance, weather, land uses, and habitat conditions. Condor 101:28-41.
Van Horne, B., G. S. Olson, R. L. Schooley, J. G. Corn, and K. P. Burnham. 1997.
Effects of drought and prolonged winter on Townsend’s Ground Squirrel demography in shrubsteppe habitats. Ecol. Monogr. 67:295-315.
42
van Woudenberg, A. M. 1992. Integrated management of Flammulated Owl breeding habitat and timber harvest in British Columbia. Masters thesis, Univ. British Columbia, Vancouver.
Verner, J. 1994. Current management situation: Flammulated Owls. Pages 10-13 in
Flammulated, Boreal, and Great Gray Owls in the United States: a technical conservation assessment (G. D. Hayward and J. Verner, eds.). USDA Forest Serv. Gen. Tech. Rep. RM-253.
White, C. M., and T. J. Cade. 1971. Cliff-nesting raptors and ravens along the
Colville River in arctic Alaska. Living Bird 10: 107-150. Wiens, J. A. 1986. Spatial scale and temporal variation in studies of shrubsteppe
birds. Pages 154-172 in Community ecology (J. Diamond and T Case, Eds.). Harper and Row Publishers, New York.
Woolfenden, G. E., and J. W. Fitzpatrick. 1984. The Florida Scrub Jay:
demography of a cooperative-breeding bird. Monogr. Pop. Biol. 20, Princeton Univ. Press, Princeton, NJ.
van Woudenberg, A. M. 1992. Integrated management of Flammulated Owl
breeding habitat and timber harvest in British Columbia. Masters thesis, Univ. British Columbia, Vancouver.
43
CHAPTER III
LIFETIME REPRODUCTION OF FLAMMULATED OWLS
Abstract. - I investigated lifetime reproduction of 22 male and 37 female adult
Flammulated Owls (Otus flammeolus) on 511 ha in Colorado from 1981-1999. Mean
(+ SE) clutch size over the study was 2.5 + 0.1 eggs and mean brood size for
successful nests was 2.4 + 0.1 owlets. Total years that individual owls bred
successfully (i.e., produced at least one owlet/yr) was 1.7 + 0.3 yr (range 1 to 7 yr, n
= 36) for females and 2.5 + 0.4 yr (range 1 to 9 yr, n = 22) for males. Individual
females produced a total 4.9 + 0.8 eggs (range 1 to 19 eggs) and 4.1 + 0.7 owlets
(range 0 to 18 owlets), and males produced 6.8 + 1.1 eggs (range 2 to 25 eggs) and
6.1 + 1.0 owlets (range 0 to 22 owlets), in their lifetimes. Relatively few individuals
accounted for a large proportion of the total production of offspring. Seventeen
percent of 37 females produced 50% of total owlets, while 27% of 22 males produced
50% of total owlets. Adults with longer breeding lifespans produced more owlets in
their lifetimes, and differences between sexes in lifetime reproduction and lifetime
number of mates were associated with greater longevity on the study area by males.
Flammulated Owls have breeding lifespans comparable to other, larger strigiforms,
and coupled with the fact that I never observed renesting or multiple broods, these
data support reports that the owl generally has a K-selected life-history strategy.
44
INTRODUCTION
Avian reproductive strategies have been shaped by a variety of environmental
factors including climate, habitat, food resources, and predators (Stearns 1976,
Southwood 1977). These factors have diversified reproductive strategies among
species by influencing parameters such as age of first breeding (e.g., Pietiainen 1988),
sex- and age- dependent reproduction (e.g., Pianka 1970, Pianka and Parker 1975,
Orians and Beletsky 1989, Fitzpatrick and Woolfenden 1989), and breeding lifespan
(e.g., Owen and Black 1989). Studying reproductive strategies is important because
they offer insight into evolutionary factors influencing reproductive rate (e.g., Lack
1947, Skutch 1949), and because this information helps determine how species may
respond to environmental changes (Newton 1998).
Reproductive strategies of species are perhaps best determined by
measurements of lifetime reproductive success (LRS), the total offspring raised by
individuals over their lifetimes. LRS, which is based on longitudinal studies,
provides more accurate estimates of reproductive contributions by individuals and of
variation in reproductive output among individuals than annual measures from cross-
sectional studies (Newton 1989a, Clutton-Brock 1988). LRS may also provide the
best estimate of biological fitness for individuals because these data elicit information
on the relative contribution of individual genotypes, and allow identification of traits
that may most contribute to fitness (Williams 1966, Newton 1989a).
LRS studies have revealed patterns in reproductive strategies and variance in
productivity among birds. Small species generally have shorter but more productive
45
breeding lives than large species. For example, the relatively small Kingfisher
(Alcedo atthis) breeds at less than one yr, seldom lives more than 4 yr, and may
produce more than 20 young per yr (Bunzel and Druke 1989). In contrast, the much
larger Barnacle Geese (Branta leucopsis) generally initiates breeding at 7 yr, and
produces no more than 10 offspring in a lifespan that may exceed 20 yr (Owen and
Black 1989). Despite ecological differences among birds, most studies of LRS have
shown that a few individuals produce a disproportionately large percentage of the
next generation (Clutton-Brock 1988, Newton 1989a). In both the Kingfisher and
Barnacle Geese, for example, fewer than one-third of the breeding population
produces 50% of all offspring (Bunzel and Druke 1989, Owen and Black 1989).
LRS is little studied among raptors because most of these species are
relatively long-lived. LRS has been estimated for Eurasian Sparrowhawk (Accipiter
nisus; Newton 1989b), Merlin (Falco columbarius; Wiklund 1995), Eastern Screech
Korpimaki 1992), and Barn Owl (Tyto alba; Marti 1997).
I present LRS data for a population of Flammulated Owls (O. flammeolus) in
Colorado studied for 19 years (1981-1999). The Flammulated Owl is a cavity-nester
that breeds in montane forests of western North America (McCallum 1994, Linkhart
et al. 1998). These owls are entirely insectivorous, feeding mostly on small moths
(Reynolds and Linkhart 1987), and are among the most migratory of all North
American strigiforms (Johnsgard 1988), breeding as far north as southern British
Columbia and wintering as far south as El Salvador (McCallum 1994).
46
Determination of LRS in Flammulated Owls is interesting because, while the owl’s
lifetime reproduction might be expected to be similar to that of other raptors (e.g.,
long breeding lifespan), it is one of the smallest North American owls and annually
migrates the greatest distance (Johnsgard 1988). Specifically, my objectives were to:
(1) describe individual variation in LRS among both sexes; (2) compare LRS
between sexes; (3) identify life-history attributes that have important influences on
LRS; and (4) compare the reproductive strategy of Flammulated Owls to other
raptors. Compared to other, larger raptors, I predicted that Flammulated Owls would
have shorter reproductive lifespans, and produce more offspring over their lifetimes.
METHODS
Study Area
The study area was a 511 ha tract on the Manitou Experimental Forest in
Teller Co., Colorado. Forests within the study area consisted of (1) ponderosa pine
(Pinus ponderosa) mixed with Douglas-fir (Pseudotsuga menziesii), generally on
ridgetops and south- and west-facing slopes, (2) quaking aspen (Populus tremuloides)
stands on lower slopes and bottoms of moist drainages, (3) quaking aspen stands
mixed with blue spruce (Picea pungens) in bottoms, lower slopes, and benches in
mesic areas, and (4) Douglas-fir mixed with blue spruce on higher slopes in drainages
and on north-facing slopes. Tree cutting on the study area has not occurred since the
1880s, when a light harvest for railroad ties occurred (Reynolds et al. 1985). Snags
and trees with cavities were relatively abundant throughout the study area (Reynolds
et al. 1985). Elevations ranged from 2,550-2,855 m.
Data Collection and Analysis
47
From 1981-1999, I collected data on the demographic performance of
Flammulated Owls on the study area. Each spring and summer, I searched the entire
study area for territorial males (Marshall 1939). Territories were identified by
marking territorial song-trees of males (Reynolds and Linkhart 1984) and using radio-
telemetry in 1982-1983 (Linkhart et al. 1998). Once territory boundaries were
delineated, I located all suitable nesting cavities (tree cavities with entrance diameters
> 4 cm) within territories and checked each for nesting owls (Reynolds and Linkhart
1984). Unpaired males typically sang throughout a breeding season, whereas singing
in nesting males dramatically declined after egg-hatch (Reynolds and Linkhart 1987).
Because I spent considerable time during each breeding season monitoring singing
males and searching for nests in their territories, I was confident that all nests were
located. Most nests were found during incubation and nests were checked at least
weekly (often two or three times per week) until the young fledged. Breeding adults
were captured at nests (occasionally on perches or day roosts) and banded with U. S.
Fish and Wildlife Service leg bands (Reynolds and Linkhart 1984). I banded owlets
when 2-3 weeks old (fledging occurs at 22-24 days; Reynolds and Linkhart 1987).
Because no nests failed beyond the midpoint of the nestling period (duration of
nestling period was 22-24 d; Reynolds et al. 1987), mean number of fledglings per
brood was identical to mean number of banding-age owlets per brood.
I determined lifetime reproduction for all males and females captured and
recaptured between 1981 and 1999 according to the following criteria. First, I
included only individuals that bred at least once during the study (territorial, non-
breeding males were rarely captured). Second, individuals whose breeding lifespans
48
included 1981 or 1999 (the first and last years of study) were included only if their
total annual breeding attempts were greater than the mean for all individuals whose
known breeding lifespans began after 1981 and terminated before 1998 (henceforth
“inclusive owls”). Mean (+ SE) total breeding attempts for inclusive adults was 2.5 +
0.5 (n = 18) for males and 1.7 + 0.3 (n = 31) for females, so this criterion excluded
seven adults—four males (two males with two breeding attempts and two with one
breeding attempt) and three females (each with one breeding attempt) from LRS
calculations. Third, breeding individuals were excluded from calculations of certain
parameters of lifetime reproduction if their identity was unknown (i.e., they were not
recaptured) in any given year or if their full breeding histories were unknown
(excepting criterion #2 above). Age of first-time breeders could not be determined.
Data on reproductive parameters are therefore conservative if owls nested outside, but
immigrated onto, the study area prior to being banded. This bias may be small given
that I documented just nine cases of breeding dispersal between 1981-1999, with
dispersers usually moving to adjacent territories (Linkhart and Reynolds 1998).
For all individual owls meeting the above criteria, I determined the following
parameters for both sexes: (1) lifetime breeding attempts, defined as the combined
total of successful (i.e., fledged at least one owlet) and failed breeding attempts; (2)
lifetime successful breeding attempts; (3) lifetime production of eggs and fledglings;
(4) relationship between total breeding years and production of fledglings; (5)
relationship between total fledglings and total eggs, (6) contribution to total offspring
by individual adults; and (7) lifetime number of new mates.
Statistical analyses were performed using SAS (SAS Institute 1995). I used
49
Wilcoxon’s test (PROC NPAR1WAY) to evaluate sex differences in reproductive
parameters and effect of breeding experience on brood sizes, and simple linear
regression (PROC GLM) to examine relationships between variables. Means are
presented + standard error (SE), and analyses were considered significant if P < 0.05.
RESULTS
Over the 19 yr study, I recorded 82 breeding attempts (i.e., at least one egg
laid) by 65 adults. I documented the reproductive lives of 59 of these adults: 22
males and 37 females. Except for four males and three females that nested in 1999,
no individuals who nested in prior years on the study area were known to be alive in
1999. Only two individuals (females) returned to breed on the study area (one in
1998 and 1999) after being absent for a breeding season; these females returned to
breed one and two territories distant from original territories (Chapter 4). Unless
otherwise noted, the following are based on these 59 adults.
Lifetime Breeding Attempts
Lifetime reproduction of individuals is the product of mean clutch or brood
size and lifetime breeding attempts. Elsewhere I reported that, over the 19 yr study
for this population, mean clutch size was 2.5 + 0.1 eggs (n = 29, range = 2–4) and
mean brood size for successful nests was 2.4 + 0.1 (n = 67, range = 1-4; Chapter 2).
Total breeding attempts is dependent upon the age of first breeding, number of
breeding attempts per year, and breeding lifespan (i.e., duration of breeding life). I
was unable to determine age of first breeding because adults could not be aged. Most
raptors produce only one brood per year, although several small species of hawk and
falcon and some medium to large owl species renest after initial breeding attempts
50
fail (Newton 1989a, Johnsgard 1988). I never documented any instances of renesting
or multiple broods in Flammulated Owls during the study, even for pairs whose nests
failed early in the breeding cycle. Since this species has a maximum of one breeding
attempt per year, lifetime breeding attempts was equivalent to breeding lifespan.
Most owls in this study bred more than once in their lifetimes, and the mean
total breeding attempts was 2.4 + 0.3 (range = 1 to 10). However, males had
significantly more mean total breeding attempts in their lifetimes (3.0 + 0.5) than
females (2.0 + 0.3; z = 2.38, P = 0.02; Fig. 8). Just 20% of females had three or more
breeding attempts compared to nearly 50% of males (Fig. 8).
Because unsuccessful breeding attempts do not contribute to an individual’s
LRS, I determined total years that owls bred successfully (i.e., produced at least one
owlet). Most owls in the population bred successfully more than once in their
lifetimes, and mean total successful nest attempts for all owls was 2.0 + 0.2 (range =
0 to 9). However, on average males bred successfully more yr (2.5 + 0.4 yr) than
females (1.7 + 0.3 yr; z = 2.46, P = 0.01; Fig. 9). Just 20% of females bred
successfully for > 3 yr compared to nearly 50% of males. These data are similar to
lifetime breeding attempts for both sexes because only 14 nests failed during the
study—82% of all breeding attempts were successful (Chapter 2). The percentage of
individuals that had greater than 80% lifetime nesting success was 73% among males
and 75% among females (Fig. 10).
Lifetime Reproduction
Most owls produced or tended nests that produced > 5 eggs in their lifetimes;
mean total production was 5.7 + 0.7 eggs. However, males tended nests that
51
Figure 8. Lifetime number of nesting attempts by female (light bars) and male (dark
bars) owls.
Per
cent
of i
ndiv
idua
ls
Years
0
10
20
30
40
50
60
70
1 2 3 4 5 6 7 8 9 10
52
Figure 9. Lifetime number of successful nests by female (light bars) and male (dark
bars) owls.
0
10
20
30
40
50
60
70
0 1 2 3 4 5 6 7 8 9
Total successful nests
Per
cent
of i
ndiv
idua
ls
53
Figure 10. Lifetime percentage of successful nests by female (light bars) and male
(dark bars) owls.
Percent of nests successful
Per
cent
of i
ndiv
idua
ls
0
20
40
60
80
100
0-20 21-40 41-60 61-80 81-100
54
produced significantly more mean eggs in their lifetimes (-x = 6.8 + 1.1) than females
produced (-x = 4.9 + 0.8; z = 2.29, P = 0.02; Fig. 11). Less than 30% of females
produced > 6 eggs while nearly 50% of males produced > 6 eggs. Yr of breeding
experience were not associated with larger clutch sizes in either sex; females breeding
for the first time on the study area had mean clutch sizes of 2.5 + 0.1 eggs (n = 29)
and females breeding for > 2 yr had clutches of 2.6 + 0.2 eggs (n = 8; z = 2.16, P =
0.03). Mean clutch size of males breeding for the first time and males breeding for >
2 yr were nearly identical (both produced clutches of 2.6 + 0.2 eggs, n = 14 and 8,
respectively; z = -0.35, P = 0.73).
Owls produced or tended nests that produced a mean 4.8 + 0.6 fledglings over
their lifetimes. As with lifetime production of eggs, however, over their lifetimes
males tended nests that produced more mean fledglings (-x = 6.1 + 1.0) than females
produced (-x = 4.1 + 0.7; z = 2.48, P = 0.01; Fig. 12). Fourteen percent of females
and 5% of males did not produce any fledglings (their nests failed), while 25% of
females and 47% of males produced > 6 fledglings in their lifetimes. Since my data
on lifetime production of eggs and fledglings do not include territorial but non-
nesting males—who comprised approximately half of all territorial males annually
and who may not produce any offspring (Chapter 4)—estimates of LRS parameters
for males may be inflated relative to females.
Previous breeding experience did not affect brood sizes of either sex. Females
breeding for the first time on the study area had mean brood size of 2.1 + 0.2 owlets
(n = 21) while females breeding for > 2 yr had broods of 2.0 + 0.3 owlets (n = 13; z =
0.39, P = 0.69). Males breeding for the first time on the study area had mean brood
55
Figure 11. Lifetime production of eggs by female (light bars) and male (dark bars)
owls.
0
5
10
15
20
25
30
35
40
45
1 3 5 7 9 11 13 15 17 19 21 23 25
Total eggs
Per
cent
of i
ndiv
idua
ls
56
Figure 12. Lifetime production of fledglings by female (light bars) and male (dark
bars) owls.
Total fledglings
Per
cent
of i
ndiv
idua
ls
0
5
10
15
20
25
30
35
1 3 5 7 9 11 13 15 17 19 21 23
57
size of 2.3 + 0.2 owlets (n = 14) while males breeding for > 2 yr years had broods of
2.2 + 0.2 owlets (n = 14; z = 0.73, P = 0.46).
Breeding lifespan (i.e., total years in which owls bred) appeared to be an
important determinant of lifetime reproduction. Breeding lifespan was positively
correlated with total production of fledglings, although the relationship was more
strongly linear for females (r = 0.96, F = 389.4, P < 0.001; Fig. 13A) than for males
(r = 0.87, F = 63.0 P = 0.001; Fig. 13B). Females produced a mean 4.1 fledglings in
2.0 breeding years, and males produced a mean 6.1 fledglings in 3.0 breeding years.
Thus, the more years that Flammulated Owls bred, the more fledglings they
produced. Total owlet production was positively correlated with total egg production
for both sexes (r = 0.97 for females; r = 0.95 for males; Fig. 14A and 14B), and the
strength of the correlation reflected the fact that relatively few nests failed.
The contribution of total offspring by individuals varied greatly for adults of
both sexes. The most productive female produced 18 fledglings over a breeding
lifespan of 8 yr, accounting for 12% of total fledglings produced during the study.
Overall, 17% of females produced 50% of total fledglings (Fig. 15A). The most
productive male tended nests that produced 22 fledglings over a breeding lifespan of
10 yr, accounting for 16% of all fledglings produced during the study. Overall, 27%
of males produced 50% of total fledglings (Fig. 15B).
Lifetime Number of New Mates
Lifetime number of new mates is defined as the total number of unique
partners with which an individual bred over his/her lifetime. Most owls had one or
two new mates in their lifetimes; number of mates for sexes combined was 1.6 + 0.1
58
Fig. 13. Relationship between total breeding years and total fledglings by (A) female
and (B) male owls. Duplicate symbols are omitted from the graph.
B
y = 1.6x + 1.1
R2 = 0.76
0
5
10
15
20
25
0 2 4 6 8 10
y = 2.3x - 0.5
R2 = 0.93
0
5
10
15
20
25
0 2 4 6 8 10
Total breeding years
Total breeding years
Tot
al fl
edgl
ings
T
otal
fled
glin
gs
A
59
Fig. 14. Relationship between total eggs and fledglings for female (A) and male (B)
owls. Duplicate symbols are omitted from the graph.
y = 0.9x + 0.3
R2 = 0.95
0
5
10
15
20
25
0 5 10 15 20 25
B
A
Tot
al fl
edgl
ings
T
otal
fled
glin
gs
Total eggs
Total eggs
y = 0.9x - 0.1
R2 = 0.97
0
5
10
15
20
0 5 10 15 20
A
60
Fig. 15. Percent of total fledglings produced by varying percentages of female (A)
and male (B) owls. Axes showing percent of males and females were based
on ordering individuals from highest to lowest productivity.
0
20
40
60
80
100
0 20 40 60 80 100
0
20
40
60
80
100
0 20 40 60 80 100
Per
cent
of t
otal
fled
glin
gs
(n =
138
) P
erce
nt o
f tot
al fl
edgl
ing
s (n
= 1
34)
Percent of females (n = 35)
Percent of males (n = 22)
B
A
61
(range = 1-5 mates). However, on average males had more mates in their lifetimes
(2.0 + 0.3) than females (1.3 + 0.1; z = -2.28, P = 0.02; Fig. 16). Just 22% of females
had > 2 breeding partners in their lifetimes compared with 50% of males. These
differences are likely associated with greater male longevity on the study area
compared to females (Chapter 2).
DISCUSSION
Reproductive Strategy
Flammulated Owls, which are the second-smallest (mass 50-60 g) North
American strigiform, have a reproductive strategy that resembles larger strigiforms
for which comparable data exist. Male Flammulated Owls have a longer mean
breeding lifespan (3.0 yr) than Tengmalm’s Owls (1.5 yr; Korpimaki 1992) and Barn
Owl (1.2 yr; Marti 1997), despite the fact that Tengmalm’s Owls and Barn Owls have
greater mass by about 200% and 800%, respectively. Mean lifetime production of
fledglings by Flammulated Owls (6.1 fledglings) was somewhat more than
Tengmalm’s Owls (5.4 fledglings; Korpimaki 1992), and Barn Owls (4.7 fledglings;
Marti 1997). For females, Flammulated Owls have a longer mean breeding lifespan
(2.0 yr) than Barn Owls (1.4 yr; Marti 1997), but shorter than the much larger Ural
Owls (4.9 yr; Saurola 1989), whose mass is about 1500% that of Flammulated Owls.
Mean lifetime production of fledglings by females is less for Flammulated Owls (4.1
fledglings) than Barn Owls (6.0 fledglings; Marti 1997) and Ural Owls (8.2
fledglings; Saurola 1989), but more than Eastern Screech Owls (2.9 fledglings;
Gehlbach 1989), whose mass is about 300% of Flammulated Owls.
62
Fig. 16. Lifetime number of new mates for female (light bars) and male (dark bars)
owls.
0
20
40
60
80
100
1 2 3 4 5
Per
cent
age
of in
divi
dual
s
Total new mates
63
Elsewhere (Chapter 2) I reported data that showed, compared to other North
American strigiforms, Flammulated Owls had (1) one of the smallest and least
variable clutch sizes, (2) one of the highest rates of nesting success, and (3)
relatively long lifespan. Coupled with the fact that lifetime production of fledglings
and breeding lifespan is similar to that of several larger strigiforms, and that I never
observed renesting or multiple broods, these data indicate that Flammulated Owls
have a life-history strategy resembling other, larger strigiforms (e.g., Tawny Owls,
Southern 1970; Ural Owls, Saurola 1989; Spotted Owls, S. occidentalis, Forsman et
al. 1984). This strategy contrasts with that of Barn Owls, whose large clutch size,
propensity for multiple clutches annually by some individuals, and short breeding
lifespans resemble many passerine species (Marti 1997).
Individual Variation in LRS
Studies of LRS have revealed three patterns regarding the extent to which
individuals contribute offspring to future generations. First, some individuals that
attempt to breed fail to produce any offspring during their lifetimes (Clutton-Brock
1988, Newton 1989a). Among all birds, the proportion of individuals that attempted
to breed but produced no offspring ranges from 5% (Blue Tits, Parus caeruleus;
Dhondt 1989) to 49% (Barnacle Geese, Owen and Black 1989). In Flammulated
Owls, 14% of females and 5% of males that attempted to breed produced no
fledglings. However, this latter percentage is underestimated, because most unpaired
males, which occupied 30-70% of territories annually (Chapter 4) could not be
captured. The proportion of unpaired males that eventually bred was unknown
(Chapter 4). Second, a small percentage of breeding adults account for the majority
64
of the total fledglings produced in the population (Clutton-Brock 1988, Newton
1989a). Among all birds, the percentage of females that accounted for 50% of total
fledglings ranges from 15% (Red-billed Gulls; Mills 1989) to 31% (Kingfishers;
Bunzel and Druke 1989), and the percentage of males ranges from 14% (Indigo
Buntings; Payne 1989) to 30% (Kingfishers; Bunzel and Druke 1989). In
Flammulated Owls, 17% of females and 27% of males accounted for 50% of total
fledglings. As noted above, the percentage of fledglings accounted for by males is
perhaps inflated. Third, a high proportion of fledglings die before ever attempting to
breed (Clutton-Brock 1988, Newton 1989a). Among all birds, the percentage of
fledglings that die before attempting to breed ranges from 42% (Barnacle Geese;
Owen and Black 1989) to 86% (Blue Tits; Dhondt 1989). I could not determine the
proportion of Flammulated Owls that died before breeding because I could not
distinguish between death and dispersal for fledglings and adults.
Sexual Variation in LRS
In many bird species the extent of sexual differences in variance of LRS is
correlated with the degree of sexual dimorphism (Newton 1989a). Species showing
the least sexual dimorphism exhibit the least sexual variance in LRS, such as in
Florida Scrub Jays (Aphelocoma c. caerulescens; Fitzpatrick and Woolfenden 1989),
while species showing the most sexual dimorphism exhibit greatest differences
between the sexes, with the most dimorphic sex exhibiting the greatest variance in
LRS such as in Red-winged Blackbirds (Agelaius phoeniceus; Orians and Beletsky
1989). Inter-sexual variation in LRS of raptors, which exhibit varying degrees of
reversed sexual size dimorphism (Snyder and Wiley 1976, Mueller 1986), has been
65
studied in only three species: Osprey, Barn Owls, and Flammulated Owls (this
study). Extent of dimorphism is similar among these species (females have 20-30%
more mass than males), but inter-sexual differences in LRS exist only within the two
owl species. Male and female Barn Owls produced a similar mean number of total
eggs and fledglings, but females had longer breeding lifespans than males (Marti
1997). In contrast, mean lifetime production of eggs and fledglings were similar
between male and female Flammulated Owls. However, males had longer breeding
lifespans and more mates over their lifetimes than females. These differences may
result from males having greater longevity than females, which is suggested by an
apparent surplus of males—only about 50% of males breed annually (Chapter 5).
The underlying reasons for differences in longevity are uncertain, but females may
suffer higher mortality than males due to the high cost of egg production immediately
following spring migration (Chapter 2). Alternatively, higher rates of breeding
dispersal by females (Chapter 4) may mean that some females may breed either prior
to, or subsequent to, their arrival on my study area.
Demographic and Ecological Correlates of LRS
My data showed that total breeding years were strongly correlated with
lifetime productivity for female (r = 0.96) and male (r = 0.87) Flammulated Owls,
because clutch sizes varied little and nesting success was high. In fact, breeding
lifespan has emerged as the major demographic determinant of LRS (Newton 1989a).
Among all birds, regression analyses have shown that variance in fledgling
production that was accounted for by breeding lifespan ranged from 29% (Barnacle
Geese; Owen and Black 1989) to 86% (Blue Tits; Dhondt 1989). For some species,
66
other important factors contributing to differences in LRS among individuals were
offspring survival between the egg and fledgling stages and lifetime fecundity
(Newton 1989a).
Few studies have examined the ecological factors associated with individual
variation in LRS. Newton (1989b) reported that LRS of European Sparrowhawks
was greatly influenced by territory quality, where better territories were characterized
as those most frequently occupied annually in a previous study. In my study,
variance in LRS also may have been associated with habitat quality. Males and
females producing the most eggs and fledglings over their lifetimes did so on four
territories (A4, A8, A11, and A29) that were, over the 19 yr study, the most
productive of all territories (Chapter 5). Productivity over all territories was
positively correlated with mature, open stands of ponderosa pine/Douglas-fir and
negatively correlated with Douglas-fir forests, which were denser and consisted of
smaller trees (Chapter 5). Variance in LRS was associated with variable food
abundance in Tengmalm’s Owl (Korpimaki 1988, 1992), nest predation in Merlins
(Falco columbarius; Wiklund 1995), and extreme weather in Great Tits (Parus
major; McCleery and Perrins 1989) and Barnacle Geese (Owen and Black 1989).
Annett and Pierotti (1999) found that long-term reproductive output in Western Gulls
(Larus occidentalis) was strongly influenced by choice of diet, with increasing
amounts of fish in the diet associated with greater survival and reproduction.
67
Literature Cited Annett, C. A. and R. Pierotti. Long-term reproductive output in Western Gulls:
consequences of alternate tactics in diet choice. Ecol. 80:288-297. Bunzel, M. and J. Druke. 1989. Kingfisher. Pages 107-117 in Lifetime reproduction
in birds (I. Newton, Ed.). Academic Press, San Diego, California. Clutton-Brock, T. H. 1988. Reproductive success. University of Chicago Press,
Chicago. Dhondt, A. A. 1989. Blue Tit. Pages 15-33 in Lifetime reproduction in birds (I.
Newton, Ed.). Academic Press, San Diego, California. Fitzpatrick, J. W. and G. E. Woolfenden. 1989. Florida Scrub Jay. Pages 201-218 in
Lifetime reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, California.
Forsman, E. D., E. C. Meslow, and H. M. Wight. 1984. Distribution and biology of
the Spotted Owl in Oregon. Wildl. Monogr. 87:1-64. Gehlbach, F. R. 1989. Screech-owl. Pages 315-326 in Lifetime reproduction in
birds (I. Newton, Ed.). Academic Press, San Diego, California. Johsnsgard, P. A. 1988. North American owls: Biology and natural history.
Smithsonian Institution Press, Washington, D.C. Korpimaki, E. 1988. Costs of reproduction and success of manipulated broods under
varying food conditions in Tengmalm’s Owls. J. Anim. Ecol. 57:97-108. Korpimaki, E. 1992. Fluctuating food abundance determines the lifetime
reproductive success of male Tengmalm’s Owls. J. Anim. Ecol. 61:103-111. Linkhart, B. D. and R. T. Reynolds. 1997. Territories of Flammulated Owls: Is
occupancy a measure of habitat quality? Pp. 250-254 in Biology and conservation of owls of the northern hemisphere (J. R. Duncan, D. H. Johnson, and T. H. Nichols, Eds.). USDA Forest Serv. Gen Tech. Rep. NC-190.
Linkhart, B. D., R. T. Reynolds, and R. A. Ryder. 1998. Home range and habitat of
breeding Flammulated Owls in Colorado. Wilson Bull. 110:342-351. Marshall, J. T., Jr. 1939. Territorial behavior of the Flammulated Screech Owl.
Condor 41:71-78.
68
Marti, C. D. 1997. Lifetime reproductive success in Barn Owls near the limit of the
species range. Auk 114:581-592. McCallum, D. A. 1994. Flammulated Owl (Otus flammeolus). In The birds of North
America, No. 93 (A. Poole and F. Gill, Eds.). Academy of Natural Sciences, Philadelphia; and American Ornithologists’ Union, Washington, D.C.
McCleery, R. H., and C M. Perrins. 1989. Great Tit. Pages 35-53 in Lifetime
reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, California.
Mills, J. A. 1989. Red-billed Gull. Pages 387-404 in Lifetime reproduction in birds
(I. Newton, Ed.). Academic Press, San Diego, California. Mueller, H. C. 1986. The evolution of reversed sexual dimorphism in owls: an
empirical analysis of possible selective factors. Wilson Bull. 98:387-406. Newton, I. 1989a. Lifetime reproduction in birds. Academic Press, San Diego,
California. Newton, I. 1989b. Sparrowhawk. Pages 279-296 in Lifetime reproduction in birds
(I. Newton, Ed.). Academic Press, San Diego, California. Newton, I. 1998. Population limitation in birds. Academic Press, New York. 597
pp. Orians, G. H. and L. D. Beletsky. 1989. Red-winged Blackbird. Pages 183-197 in
Lifetime reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, California.
Owen, M., and J. M. Black. 1989. Barnacle Goose. Pages 349-362 in Lifetime
reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, California.
Payne, R. B. 1989. Indigo Bunting. Pages 153-172 in Lifetime reproduction in birds
(I. Newton, Ed.). Academic Press, San Diego, California. Pianka, E. R. and W. S. Parker. 1975. Age-specific reproductive tactics. Amer.
Natural. 109:453-464. Pietiainen, H. 1970. Breeding season, age, and the effect of experience on the
reproductive success of the Ural Owl (Strix uralensis). Auk 105:316-324. Postupalsky, S. 1989. Osprey. Pages 297-313 in Lifetime reproduction in birds (I.
Newton, Ed.). Academic Press, San Diego, California.
69
Reynolds, R. T. and B. D. Linkhart. 1984. Methods and materials for capturing and
monitoring Flammulated Owls. Great Basin Nat. 44:49-51. Reynolds, R. T., B. D. Linkhart, and J. Jeanson. 1985. Characteristics of snags and
trees containing cavities in a Colorado conifer forest. USDA Forest Serv. Res. Note RM-455. 6 pp.
Reynolds, R. T. and B. D. Linkhart. 1987. The nesting biology of Flammulated
Owls in Colorado. Pp. 239-248 in Biology and conservation of northern forest owls (R. W. Nero, R. J. Clark, R. J. Knapton, and R. H. Hamre, Eds.). USDA Forest Serv. Gen. Tech. Rep. RM-142.
SAS Institute, Inc. 1995. SAS user’s guide, SAS Institute, Inc. Cary, North
Carolina. Saurola, P. 1989. Ural Owl. Pages 327-345 in Lifetime reproduction in birds (I.
Newton, Ed.). Academic Press, San Diego, California. Skutch, A. F. 1949. Do tropical birds rear as many young as they can nourish? Ibis
91:430-455. Snyder, N. F. and J. W. Wiley. 1976. Sexual size dimorphism in hawks and owls of
North America. Ornith. Monogr. 20:1-96. Southwood, T. R. E. 1977. Habitat, the templet for ecological strategies? J. Anim.
Ecol. 46:337-365. Stearns, S. C. 1976. Life history tactics, a review of the ideas. Quart. Rev. of Biol.
51:3-47. Southern, H. N. 1970. The natural control of a population of Tawny Owls (Strix
aluco). J. Zool. (London) 162:197-285. Wiklund, C. G. 1995. Nest predation and life-span: components of variance in LRS
among Merlin females. Ecol. 76:1994-1996. Williams, G. C. 1966. Natural selection, the costs of reproduction, and a refinement
of Lack’s Principle. Amer. Natural. 100:687-690.
70
CHAPTER IV
MATE AND SITE FIDELITY AND BREEDING DISPERSAL IN
FLAMMULATED OWLS
Abstract. - I investigated territory and mate fidelity and dispersal in a migratory
population of Flammulated Owls (Otus flammeolus) on 511 ha in central Colorado. Over
the 19 yr (1981-1999) study, a mean 8.0 + 0.5 (SE) territories were occupied annually by
breeding pairs or unpaired males. Rate of return to the study area was higher for males
(59%) than for females (37%), and mean tenure on territories was nearly twice as long for
males (3.0 + 0.5 yr) as for females (1.6 + 0.2 yr). Males also showed greater annual
territory fidelity than females; 98% of returning males stayed on original territories
compared to 78% of females. Failure of previous year’s nest and breeding status (paired
vs unpaired) were associated with reduced territory fidelity in females but not males,
while return of previous mate did not affect territory fidelity for either sex. Mean pair
duration was 1.4 + 0.1 yr and mate fidelity was high; 96% of pairs retained the same mate
when both mates returned from migration. Reproductive success was not correlated with
length of pair bond; annual initiation of egg-laying and productivity (mean
fledglings/brood) were similar for pairs that bred two or more years and pairs that bred for
the first time. I documented one instance of natal dispersal. Breeding dispersal was
female-biased; 8 of 9 dispersals were by females that moved one or two territories away
from their original territories. Females whose nests failed the previous year had lower
return rates to the study area than females whose previous nests were successful.
71
Consequently, dispersal distance may be bimodal with females dispersing longer distances
after nesting failure and shorter distances after successful nests. Dispersal was not
correlated with previous breeding experience, previous nest failure, mate quality, and
productivity, but females often dispersed when mates did not return and they dispersed to
territories on which total productivity over the study was higher than on original
territories.
72
INTRODUCTION
In stable environments many birds annually return to the same breeding sites
and retain the same breeding partners (Greenwood 1980, Black 1996). Particularly
for migratory species, site fidelity may confer benefits that include better knowledge
of locations for foraging, nesting, and escaping predators, and improved chance of
maintaining a breeding territory and mating (Hinde 1956, Greenwood 1980, Shields
1984, Part 1994, 1995). If neighbors return, site-faithful individuals also benefit by
avoiding initial cost of contesting territory boundaries with unfamiliar individuals
(Krebs 1982, Maynard Smith 1982). Mate fidelity is thought to improve coordination
and cooperation between mates, prolong biparental investment, and reduce costs
associated with mate sampling such as risk of predation or failing to find a suitable
mate in time to breed (Black 1996). Studies have shown that mates who bred
together previously had larger clutches and higher nest success than newly formed
pairs (Mills 1973, Coulson and Thomas 1983, Newton and Marquiss 1982,
Korpimaki 1988, Bradley et al. 1990, Orell et al. 1994, Murphy 1996).
In spite of benefits, mate and site fidelity involve some costs. Site faithfulness
increases the inbreeding probability, and may lower chances for reproductive success
due to decrease in habitat quality resulting from habitat changes (i.e., succession or
disturbance), predation, or competition (Greenwood et al. 1978, Oring and Lank
1982). For migratory species, costs of mate fidelity may include waiting for mates to
return or searching for new mates (Black 1996). When costs associated with mate or
site fidelity exceed benefits, individuals should disperse. In fact, several studies have
shown that fitness of individuals increased following breeding dispersal, when adults
73
moved to new breeding sites (Shields 1984, Payne and Payne 1993, Part 1995, Forero
et al. 1999).
Despite much study the past two decades, avian fidelity and dispersal are still
poorly understood primarily because investigations require gathering longitudinal
data on marked individuals and studying sufficiently large areas to document longer-
distance dispersal (Paradis et al. 1998, Forero et al. 1999). These problems are the
primary reasons that raptors, which are relatively long-lived and disperse over large
areas, have been little-studied. Diurnal species studied five or more years include
Merlins (Falco columbarius; Warkentin et al. 1991), European Sparrowhawks
(Accipiter nisus; Newton and Marquiss 1982, Newton 1993), and Black Kites (Milvus
migrans; Forero et al. 1999), and nocturnal species include Tengmalm’s Owl
raptors such as Flammulated Owls where males are the primary foragers. While
longer lifespan in males has not been substantiated, it is plausible for two reasons.
First, mean longevity on the study area from 1981-1999 was significantly greater for
males than females (3.2 + 0.6 yr vs 2.0 + 0.3 yr; Chapter 2), although this disparity
may in part result from female-biased dispersal (see below). Second, unpaired males
annually occupied 10-70% of territories, which suggests a shortage of females in the
breeding population and may reflect a longer mean lifespan by males (Chapter 2).
Other studies of monogamous species have reported or suggested male-biased sex
ratios in adults (Shields 1984, Breitwisch 1989 and sources therein, Burke and Nol
1998, Gibbs and Faaborg 1990, Payne and Payne 1990, Murphy 1996, but see
Kenward 1999).
Breeding Dispersal
Female Flammulated Owls had a higher breeding dispersal rate than males
(0.22 vs 0.02), a pattern also found in many other birds including raptors (e.g.,
Greenwood 1980, Marquiss and Newton 1982, Forero et al. 1999). In all cases of
female dispersal except two (the one divorce excluded) I was able to determine that
the original male did not return. However, 4 of 6 (67%) females whose mates were
known not to return remained on original territories, suggesting that other factors
were involved. Dispersal by females when mates do not return is likely adaptive
because opportunity to breed may be lost while waiting for a new male, especially
since most females apparently return from spring migration after males. Individuals
of other species, including raptors, also dispersed when mates did not return
93
(Greenwood and Harvey 1976, Newton and Marquiss 1982, Warkentin et al. 1991,
Montalvo and Potti 1992, Forero et al. 1999).
Frequency and distance of breeding dispersal can be underestimated in small
study areas (Barrowclough 1978, van Noordwijk 1984, Koenig et al. 1996), as they
may have been in my study. Owls dispersed one or two territories away from original
territories, but half (7 of 14) of all territories were within one mean territory-diameter
(439 + 77 m), and all territories were within two mean territory-diameters, of a study
boundary (Fig. 17). Thus, owls that may have dispersed to territories farther than two
territories were less likely to be detected. Moreover, females (but not males) whose
nests failed the previous year had lower return rates (9%) to the study area than
females whose previous nests were successful (54%), suggesting either higher
mortality or higher dispersal (beyond the study area) among females whose nests
failed. Females whose prior nests failed did not appear to be in poorer physical
condition than females whose nests were successful. That females dispersed rather
than died following failed nests is supported by Haas (1998), who found that
American Robins (Turdus migratorius) and Brown Thrashers (Toxostoma rufum)
subjected to experimental nesting failure returned at significantly lower rates than
birds that had nested successfully. Indeed, other studies have found that following
nesting failure females returned less frequently to territories or they dispersed farther
than males (e.g., Shields 1984, Gavin and Bollinger 1988, Payne and Payne 1993,
Beletsky and Orians 1991, Murphy 1996).
Consequently, dispersal distance by female Flammulated Owls may be
bimodal with females dispersing longer distances following unsuccessful nests and
94
shorter distances following successful nests. Dispersal to nearby territories, as found
in many other studies (e.g., Payne and Payne 1996, Hannon and Martin 1996,
Williams 1996, Forero et al. 1999), may be beneficial because dispersers can best
judge quality of resources and owls in adjacent territories (Hinde 1956, Greenwood
1980, Ens et al. 1996). Means by which owls assess territories or mates for potential
future occupancy or pairing is uncertain but may be accomplished by extra-territory
movements by males and females (Reynolds and Linkhart 1990, BDL, unpubl. data).
Although dispersal following nesting failure is beneficial if chances of future nesting
success are improved (Murphy 1996), benefits of dispersing to more distant
territories, where owls are unlikely to have knowledge of resources or potential
mates, are not clear.
Dispersing owls moved to territories where productivity over the 19 yr study
was significantly greater than on territories from which they dispersed. Since mean
brood size of owls after they dispersed did not increase on new territories, and
because mean total owlets produced by new mates was not significantly greater than
total owlets produced by original mates, my ability to detect consequences of
breeding dispersal may require reproductive data collected over lifetimes for
particular individuals. I reported previously that long-term productivity and
occupancy of territories by breeding pairs was positively correlated with percentage
of old ponderosa pine/Douglas-fir forests and negatively correlated with percentage
of young Douglas-fir/blue spruce forests, and that old ponderosa pine/Douglas-fir
forests were used significantly more for foraging by radio-tagged males (Linkhart and
Reynolds 1997, Linkhart et al. 1998, Chapter 5). Thus, owls dispersed to territories
95
containing more old ponderosa pine/Douglas-fir and less young Douglas-fir/blue
spruce. Other raptors and passerines also dispersed to higher-quality territories, as
inferred by movements to territories having higher historical nesting success, better
prey resources, or lower risk of predation (Baeyens 1981, Marquiss and Newton
1982, Weatherhood and Boak 1986, Beletsky and Orians 1987, Matthysen 1990,
Forero et al. 1999).
Several avian studies have found that younger individuals were more likely to
disperse than older individuals (e.g., Beletsky and Orians 1987, Payne and Payne
1993, Badyaev and Faust 1996). However, breeding dispersal was not associated
with younger owls in this study. Most (5 of 8 females and 1 male) dispersing
Flammulated Owls had at > 2 yr of breeding experience.
Mate Fidelity
The high mate fidelity in Flammulated Owls contrasts with the pattern of
lower mate fidelity in most other migratory birds, which presumably occurs because
of difficulty maintaining pair bonds during migration and asynchronous arrival times
on breeding grounds compared to residents (Wickler and Seibt 1983, Murphy 1996).
That I documented only one divorce in this study suggests that mate (and possibly
territory) familiarity conferred advantages to breeding Flammulated Owls. Studies of
raptors and other birds reported that mate fidelity was associated with higher
productivity, nest success, or earlier nesting (e.g., Newton 1982, Korpimaki 1988,
Orell et al. 1994, Murphy 1996), although other species showed no apparent benefit
by being faithful to mates (e.g., Freed 1987, Warkentin et al. 1991). Flammulated
Owls paired together for > 2 yr did not initiate incubation significantly earlier nor
96
were broods significantly larger than pairs breeding for the first time, although I could
not determine if owls gained breeding experience before their tenure on the study
area. Alternatively, owls may have long-term pair bonds simply because they are
constrained from other alternatives (Freed 1987); male choice appears to be restricted
by female availability while female choice may be limited to males available in
neighboring territories with previous breeding experience. High mate fidelity also
may be facilitated by high territory fidelity by males (Murphy 1996). Indeed, mean
tenure of males on territories (3.0 + 0.5 yr) was nearly twice mean tenure of females
on territories (1.6 + 0.2 yr), suggesting that returning females were likely to find their
original males on territories. That mean pair duration over the study (1.4 + 0.7 yr)
was approximately the same as mean tenure on territories by females suggests that
pair duration was limited by territory tenure of females.
Habitat Selection
Most male Flammulated Owls occupied a single territory their entire known
reproductive lives; only one breeding male changed territories in 62 male-years.
Moreover, males often continued to occupy original territories despite being unpaired
up to four consecutive years. Recall that mean tenure for males was calculated for
males that nested at least once. These males occupied a mean 58% (+ 3%) of all
territories annually (Chapter 2). Because I rarely captured unpaired males, I was
unable to determine their tenure on territories or extent to which they may have
dispersed. However, observation of several unpaired males, distinguished by unique
vocal characteristics (e.g., song pitch), indicated that most reoccupied their original
territories following return from migration (pers. observ.). An exception was an
97
unpaired male that occupied three different territories over four breeding seasons
(pers. observ.). Given that females (new and returning) only occupied half of all
territories annually and that breeding generally occurred on the same territories each
year (Chapter 5), many newly arriving males may settle on, and commit their
reproductive lives to, territories where breeding occurs irregularly. Consequently,
high site fidelity by males may counter predictions based on habitat selection models
that assume animals select habitats conferring highest reproductive success, and if
higher-quality habitats become available individuals should move to the new sites
(Fretwell and Lucas 1969). Thus, territory fidelity by Flammulated Owls may be
considered a suboptimal form of habitat selection with respect to territory quality
(Switzer 1993). It may be more profitable for a male of a long-lived species such as
the Flammulated Owl to await the probable arrival of a female in a territory where he
is familiar with the location of important nesting resources (see sources cited in
Introduction; Chapter 5), and possibly engage in extra-pair copulations when mates
are unavailable (Reynolds and Linkhart 1990). Males of other species including
raptors have been documented remaining on a single territory, even when higher-
quality territories apparently were available (Krebs 1971, Best 1977, Searcy 1979,
Bedard and LaPointe 1984, Janes 1984, Woolfenden and Fitzpatrick 1984, Lanyon
and Thompson 1986, Korpimaki 1988).
98
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CHAPTER V
DETERMINING HABITAT QUALITY FROM LONG-TERM DEMOGRAPHICS IN
BREEDING FLAMMULATED OWLS
Abstract. – Basing the determination of habitat quality on long-term demography is
generally regarded as a valuable approach for understanding how animal populations
use space, but it has little empirical support. From 1981-1999, I measured
demographic performance of Flammulated Owls (Otus flammeolus) in a 511 ha study
area of mixed conifer forest in central Colorado. Boundaries of 12 owl territories
remained generally stable over the study, enabling me to determine demographic
performance for territories rather than for individuals. I used demographic
parameters that distinguished among territories to infer relative territory quality so
that habitat conditions could be compared across territories and with non-territory
habitat. Territories differed in total breeding yr, because some territories usually were
occupied by breeding pairs annually while other territories usually were occupied by
bachelor males. Productivity varied among territories, ranging from 0 to 35 owlets.
Mean territory tenure, which I used as an estimate of survival, and pair duration did
not differ among territories. Breeding dispersal resulted in females moving to
territories where productivity was significantly higher.
Productivity was positively correlated with territory area in ponderosa
pine/Douglas-fir forests, and with greater crown volume in the second-largest (33.0 –
48.2 cm) of four tree dbh (diameter at breast height) categories. Productivity was not
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correlated with density of cavity-trees. However, cavity trees clearly distinguished
territory from non-territory habitat, given that non-territory habitat contained < 10%
of mean cavity-tree density within territories. Few structural characteristics
distinguished combined territories from non-territory habitat. In comparisons of non-
territory habitat with three classes of territory distinguished by differing productivity,
only the high-productivity class (consisting of just one territory) contained greater
tree density and basal area in the larger dbh categories. Based on the fact that
moderate-productivity territories showed no differences in forest structure from
unoccupied habitat, and that low-productivity territories (usually occupied by
bachelor males) actually contained denser forests and smaller trees than non-territory
habitat, at least some portions of non-territory habitat may have been suitable for
territory establishment except for scarcity of cavity trees. Monitoring of predation on
artificial nests for 1 yr and relative prey abundance for 2 yr revealed no patterns
among selected territories differing in productivity, suggesting these factors were not
associated with habitat quality.
My results indicate that cavity-tree availability primarily determined where
owls established territories, while forest type and structure determined whether a
territory was more often occupied by breeding pairs or by bachelor males. High-
quality breeding habitat for Flammulated Owls in this study was characterized as
mature, relatively open stands of ponderosa pine/Douglas-fir that contained sufficient
cavity trees for nesting.
Habitat correlations with bachelor yr differed markedly from correlations with
productivity, indicating that inferring habitat quality based on abundance or duration
107
of territory occupancy by males would be misleading. Breeding year may be a good
surrogate for productivity in future efforts to identify important breeding habitats for
this species, at least where other demographic parameters such as nesting success are
similar to mine.
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INTRODUCTION
Birds often occupy a range of habitats differing in resources necessary for
reproduction and survival (Fretwell and Lucas 1969, Cody 1985). These resources
include food, foraging sites, nesting sites, and places to avoid predators or
aspen/blue spruce; and (6) Douglas-fir/limber pine (Figure 3). Overall model
accuracy was 83%, based on comparing field-assigned and analysis-assigned forest
types at sampling points. I then used ArcView (ESRI 1995) to map the six forest
types and determine area (ha) and percentage of area in each forest type for the study
area, and area (ha) and percentage of area of forest types in each territory.
Comparison of Structure Among Forest Types
To compare forest structure (overstory and understory variables; Table 4)
among disproportionately sampled forest types, I weighted individual sampling points
proportional to amount of area in each forest type and then pooled these weighted
values within each forest type and calculated from these a mean forest-type value for
each habitat variable. This is similar to estimation for stratified random sampling
(Cochran 1977), with the overall sample size maintained at the total number of
sampling points:
Wx= (a1/at)nt / n1 (Equation 1)
where, Wx was the weight factor for sampling points in forest type x, a1 was area
occupied by forest type x, at was the total area occupied by all forest types, nt was the
total number of sampling points, and n1 was the number of sampling points in forest
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type x. I then used ANOVA and Tukey’s Test (PROC GLM; SAS 1996) to compare
specific forest structure variables (Table 1) among the six forest types.
Comparisons Between Vegetation And Owl Demography
Determination of territory habitat was based on all sampling points within
combined territories. Boundaries of most territories remained fixed over the study
(Fig. 18). For four territories (A4, A8, A11, and A29) whose boundaries shifted
during 1983-1987, I evaluated habitat variables based on boundaries after the shifts
occurred, since new boundaries remained unchanged for the remainder of the study
and accounted for the majority (63-84%) of total study yr. I omitted two territories
(A15 and A24) from analyses of habitat quality because these territories were not
occupied after 1984 and much of the area within their boundaries was incorporated
into adjacent territories. I based determination of non-territory habitat on all
sampling points located outside of territory boundaries.
Relationship between forest type and structure and owl demography.—To assess the
relationship between forest type and demographic performance across territories, I
determined the area (ha) and proportion of total area of forest types within owl
territories using ArcView (ESRI 1996). I used Pearson product correlation analyses
(PROC REG; SAS 1996) to examine correlations among forest types and
demographic performance.
To evaluate relationships between forest structure (forest and understory
variables; Table 1) and demographic performance across territories, I weighted
individual sampling points proportional to amount of area in each forest type within a
particular territory using Equation 1. I then pooled these weighted values within each
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territory and calculated from these a mean territory value for each habitat variable. I
used Pearson product correlation analyses (PROC REG; SAS 1996) to examine
correlations between forest structure variables and demographic variables. I then
used stepwise regression analyses (PROC REG; SAS 1996) to evaluate the inter-
correlations among forest structure variables found significantly correlated with
demographic variables, and to address the increased probability of incurring Type 1
errors associated with multiple univariate comparisons.
Comparison of territory and non-territory habitat.—I compared forest structure in
territory vs non-territory habitat in two ways. First, I assessed whether forest
structure for combined territories differed from non-territory habitat. I weighted
sampling points to account for under- or over-sampling of each forest type within
combined territories or non-territory habitat using Equation 1. I used ANOVA
(PROC GLM; SAS 1996) to compare territory vs non-territory habitat for specific
variables (Table 4).
To further determine if forest structure differed between territory and non-
territory habitat, I compared non-territory habitat to three classes of territories, based
on the range in total owlets over the study (see Results): high-productivity (> 23
owlets), moderate-productivity (13 - 23 owlets), and low-productivity (< 8 owlets). I
defined these classes based on natural "breaks" in the range of total owlets; 12 owlets
separated high-productivity and moderate-productivity habitat classes, and 5 owlets
separated the moderate-productivity and low-productivity habitat classes. Although
this approach resulted in the high-productivity class containing only one territory
(A4) compared to the moderate-productivity (5 territories) and low-productivity (6
125
territories) classes, I evaluated this territory separately because preliminary analyses
indicated its forest structure differed significantly from all other territories.
To compare forest structure in the three classes of owl habitat with non-
territory habitat, within each habitat class I weighted sampling points to account for
under- or over-sampling of each forest type using Equation 1. I used ANOVA and
Tukey’s Test (PROC GLM; SAS 1996) to compare specific forest structure variables
(Table 4) between territory and non-territory habitat.
Possible Limiting Factors Associated With Habitat Relationships
Density of Cavity Trees.—I annually located all snags (standing, dead trees) and live
trees with suitable cavities for nesting (hereafter, cavity trees) within territories.
Beginning in 1983, as part of a long-term investigation to determine the spatial and
temporal dynamics of cavity trees (Reynolds et al. 1985), I also mapped the location
of cavity trees outside of territories.
Quaking aspen cavity-trees, which accounted for the majority (> 80%) of owl
nests annually (unpubl. data), typically had shorter longevity (snags 1-10 yr, live trees
5-15 yr) than conifer cavity-trees (snags > 10 yr, live trees > 20 yr; pers. observ.).
However, the general location (i.e., forest patch) and relative density of deciduous
cavity-trees changed little from 1981-1999. Given the observed constancy in spatial
dynamics and density of conifer and deciduous cavity-trees over time, I used the
location of cavity trees in 1999 as the basis for inferring general location and density
of cavity trees over the19 yr study. I mapped the locations of all cavity trees found in
1999 on a DEM layer in ArcView (ESRI 1995), and determined the density of cavity
trees within territories and outside territories. I assessed correlation between density
126
of cavity trees and demographic performance on territories with Pearson product
correlation analysis (PROC REG; SAS 1996).
Nest Predation.—Effects of nest predation were determined in two ways. First, I
monitored predation of artificial nests in six owl territories during summer 1999: two
territories producing the most owlets over the 19 yr study, two territories producing
the least owlets, and two territories producing an intermediate number of owlets. Red
Squirrels (Tamiasciurus hudsonicus) were inferred as the primary predator of owl
nests because I observed no other avian or mammalian predators of tree cavities over
the study (but see Linkhart and Reynolds 1994). I determined sites for six artificial
nests in each territory by placing two nests in each of the three most common forest
types: ponderosa pine/Douglas-fir, Douglas-fir, and quaking aspen/blue spruce.
Where possible, I used unoccupied, natural tree cavities with entrance diameters > 4
cm. In territories lacking sufficient natural cavities, I attached wooden nest boxes (40
x 20 x 20 cm, entrance 6 cm in diameter) to live trees 3-5 m above ground. Boxes
had a southern exposure, which approximated the orientation of most owl nests
(unpubl. data), and contained 1-3 cm of partially decomposed woody debris. Of 36
total artificial nests, 18 were in nest boxes. Most territories contained 3 nest boxes, 2
of which were generally in Douglas-fir forests because these forests contained the
fewest natural cavities. I placed two fresh Bobwhite Quail (Colinus virginianus) eggs
in each artificial nest beginning 10 June, when owl eggs were being incubated, and
monitored weekly predation rates until 25 July, when all but one owl brood had
fledged. Eggs lost to predation were replaced weekly, as well as eggs that remained
in nests beyond two weeks. Differences in predation rates of artificial nests among
127
territories and among forest types were determined with chi-square (PROC GLM),
and correlations between predation rates and productivity on territories were
determined with Pearson product correlation analysis (PROC REG; SAS 1996).
Second, during summer 1999 I estimated relative density of Red Squirrels in
the six owl territories where I monitored predation of artificial nests (see above).
Since Red Squirrel territories typically contain one large midden, which contain
caches of conifer cones for winter food supplies (Gurnell 1984, Hurly and Lourie
1997), I used density of large middens as a measure of relative squirrel density in owl
territories. I mapped all large middens (dimensions > 2 x 3 m, depth > 20 cm) on a
DEM in Arcview (ESRI 1995). I assessed differences in midden density among
territories with ANOVA, and differences among forest types with Tukey’s Tests
(PROC GLM; SAS 1996). Correlations between midden density and nest-predation
rates, and between midden density and demographic performance, were determined
with Pearson product correlation analysis (PROC REG; SAS 1996).
Relative Prey Abundance.—I used black-light traps (Southwood 1981) to estimate
relative arthropod abundance in two owl territories, one producing the most owlets
(A4) and one producing the fewest owlets (A18) over the study. One black-light trap
was placed in the interior of the largest available forest patches of ponderosa
pine/Douglas-fir, Douglas-fir, and quaking aspen/blue spruce. Black-light traps were
placed between adjacent trees (3-4 m apart) and hung 1.5 m above ground adjacent to
a vertical white cloth sheet (1.5 x 2.0 m). On sampling nights (see below), arthropods
that landed on the sheet were counted every quarter-hour from 2100-2300 hr and
averaged over the 2 hr. I counted all flying arthropods, mostly lepidopterans, whose
128
length from base of antennae to tip of wing was 15-29 mm. Lepidopterans of this
size were the primary prey delivered by males to nests (Linkhart et al. 1998, unpubl.
data).
I sampled arthropod populations in 1998 and 1999 during two owl nesting
stages: incubation (18 May-11 June), and nestling (23 June-23 July). I compared
arthropod abundance between the two territories by simultaneously sampling the
same forest type (e.g., ponderosa pine/Douglas-fir) in both territories. Three
sampling nights were required to make all three intra-forest type comparisons
between territories. I compared the 2 territories during 4 trap-nights (i.e., 4 nights in
each of the forest types) of the incubation stage (2 nights each in 1998 and 1999), and
5 trap-nights of the nestling stage (3 nights in 1998 and 2 nights in 1999).
Differences in relative prey abundance between territories were determined with
ANOVA (PROC GLM; SAS 1996). For these and all other statistical analyses, I
determined whether distributions of variables deviated significantly from normality
(PROC UNIVARIATE; SAS 1996), and if necessary performed data transformations
(log, square root, and arcsin) to achieve normality and reran the procedure. I use a
significance level of P = 0.05 and present means + standard error (SE).
RESULTS
Temporal/Spatial Constancy of Territories
I monitored occupancy on 14 owl territories from 1981-1999 (Fig. 1).
Territories were generally constant in time and space despite individual turnover on
each territory, with a few exceptions. First, the A24 territory was only occupied from
1981-1983. In 1984, a new male in an adjacent territory (A29) expanded the
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boundaries of his territory to include much of the western portion of A24 territory,
and these new boundaries did not change over the remainder of the study (Fig. 18).
Second, the male in A4 territory expanded the boundaries of his territory in 1983 to
include much of the eastern portion of A15 territory, which was only occupied in
1981-1982 and 1984 (Linkhart et al. 1998; Fig. 18). I did not know the boundaries of
either territory in 1984, when only A15 territory was occupied by a breeding pair, but
for the remainder of the study A4 territory contained the eastern half of A15 territory.
Finally, in 1988 the male in A8 territory expanded his territory north into the southern
portion of A11 territory, after the male in A11 territory apparently did not return from
migration, and northeast into the northern portion of A15 territory (Fig. 18). After
1988, A11 territory contained only the northern portion of the original A11 territory
(Fig. 18). In each of the above instances, shifts in territory boundaries occurred when
> 1 males in adjacent territories did not return from migration. Boundaries of all
other territories remained unchanged over the study. Males generally returned to
territories annually; territory fidelity was 98% and annual turnover was 10% (Chapter
4). Consequently, newly arriving males in the spring typically filled geographic voids
left by predecessors. The overall high stability of territory boundaries allowed me to
compare their habitat characteristics to their demographic performance over the entire
study. Below I report demographic performance for the 12 territories (omitting A15
and A24) active over the 19 yr.
Demographic Performance on Territories Territory Occupancy.—Territories differed in total yr (occupied yr) they were
occupied by owls, ranging from 3-19 yr (χ2 = 17.3, df = 10, P = 0.07; Table 5).
Table 5. Demography on owl territories from 1981-1999.
Territory Occupancy Reproductive Success
Territory
Occupied yr
Bachelor yr Breeding yra
No. success. nestsb
Nest success
(%)b
Owlets Mean owlets brood-1
Mean owlets
yr-1
A4 19 3 16 15 93.8 35 2.2 1.8
A8 16 5 11 9 81.8 23 2.1 1.2
A29 16 4 12 7 70c 17 1.7 0.9
A10 13 4 9 5 62.5c 13 1.4 0.7
A11 14 6 8 7 87.5 17 2.1 0.9
A13 12 5 7 7 100 14 2.0 0.7
A12 14 10 4 3 100c 7 1.8 0.4
A27 9 7 2 2 100 5 2.5 0.3
A20 6 3 3 3 100 8 2.7 0.4
A7 9 6 3 1 33.3 3 1 0.2
A18 13 10 3 1 100c 2 1 0.1
A2 4 3 1 0 0 0 0 0
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MEAN 12.1 5.5 6.6 5.0 77.4 12.0 1.7 0.6
SE 1.3 0.7 1.4 1.2 9.2 2.9 0.2 0.1 a Identical to nesting attempts, since owls only attempted to breed a maximum of once yr-1 and did not renest if nests failed. b Nests that fledged at least one owlet c Excludes nesting attempts where outcome of nest was unknown or nest failed due to anthropogenic causes: A29-2 attempts; A10-1
Territory quality of Flammulated Owls in this study was associated with
mature forests of ponderosa pine/Douglas-fir. Owlets and breeding yr (equivalent to
total nesting attempts), which differed markedly among territories, were positively
correlated with territory area and proportion of area in ponderosa pine/Douglas-fir
forests. Territory quality was also associated with overstory structure in these forests,
based on the fact that owlets were positively correlated with greater crown volume in
the second-largest (33.0-48.2 cm) dbh category. In contrast, Douglas-fir forests were
sub-optimal habitat for breeding owls. Owlets and breeding yr were negatively
correlated with proportion of area in these forests, which contained greater tree
density, basal area, and crown volume in the two smallest dbh categories (2.5-17.7
and 17.8-32.9 cm) than ponderosa pine/Douglas-fir. Moreover, bachelor yr (total yr
territories were occupied by unpaired males) was positively correlated with this forest
type and with basal area. Previous studies found that unpaired males of other species
including raptors occupied territories in sub-optimal habitats (Newton and Marquiss
1976, Korpimaki 1988, Burke and Nol 1998).
Density of cavity trees was not correlated with reproductive success,
indicating that abundance of cavity trees was not associated with territory quality. All
territories contained > 3 cavity trees and had > 1 breeding attempt over the study,
suggesting that relatively few cavity trees were sufficient for nesting. However,
density of cavity trees clearly distinguished territory from non-territory habitat. Non-
territory habitat not only had a cavity-tree density that was < 10% of mean density
160
within territories, but because cavity-trees generally had a clustered distribution
across the study area, most of the non-territory habitat was characterized as
containing no cavity-trees.
The apparent importance of cavity trees in territory occupancy by males was
underscored by the few differences in other aspects of forest structure that
distinguished territory from non-territory habitat overall. Compared to non-territory
habitat, combined territories contained fewer trees and less basal area in the second-
smallest dbh category, and more understory cover 5-49 cm tall. However, in
comparisons of the three classes of territory productivity (high-, moderate-, and low-
productivity) with non-territory habitat, only the high-productivity class (which
contained only A4 territory) was distinguished by having less basal area and lower
tree density in the smaller dbh categories and the converse in the larger dbh
categories. These data indicate that differences between combined territories and
non-territory habitat were attributable to the unique characterisitics of A4 territory,
and to differences between the two statistical analyses (compared with the two-class
analyses, the four-class analyses had a larger mean squared error and a higher critical
value for evaluation). Based on the fact that moderate-productivity territories showed
no differences in overstory structure from non-territory habitat, and that low-
productivity territories actually contained greater basal area and tree density in the
smaller dbh categories than non-territory habitat, availability of cavity trees appeared
fundamentally important for territory establishment by males. These data also
suggest that at least some portions of non-territory habitat, which overall contained
similar proportions of each forest type to combined territories, were potentially
161
suitable for occupancy by males except for scarcity of cavity trees. Thus, despite the
fact that snags and cavity trees were relatively abundant on the study area, which had
not been harvested since the 1800s (Reynolds et al. 1985), cavity-tree availability
clearly affected owl distribution and density, as it has with many other secondary-
cavity nesters (e.g., Brawn and Balda 1988, Newton 1998 and sources cited therein).
In summary, my results indicate that habitat quality was determined by two
primary factors. First, cavity-tree availability determined where owls established
territories, and second, forest type and structure determined whether a territory was
more often occupied by breeding pairs or by bachelor males. High-quality breeding
habitat for Flammulated Owls in this study was characterized as mature, relatively
open stands of ponderosa pine/Douglas-fir that contained sufficient cavity trees for
nesting.
Uniqueness of A4 Territory
A4 territory was by far the most productive (35 owlets, 16 breeding yr) of all
territories, and contained significantly greater basal area, crown volume, and tree
density in the second-largest dbh category than the moderate and low-productivity
classes. Was the habitat in A4 territory optimal for Flammulated Owls? Given its
uniqueness I cannot be certain, but several observations indicate this may be true.
First, correlations between reproductive success on A4 territory and forest type and
forest structure represented a positive linear extension of patterns across all other
territories. Second, duration of this study was likely sufficient to dilute chance effects
associated with quality of individual owls. In fact, A4 territory was occupied by 8
unique females and 5 unique males over the 19 yr study, both of which were more
162
than any other territory. Finally, A4 territory was the recipient of the most dispersals
(3; 2 females and 1 male), suggesting that this territory was preferable for breeding.
Possible Factors Underlying Habitat Relationships
Nest predation is a primary cause of nesting mortality for many bird species
(e.g., Skutch 1949, Ricklefs 1969), and is believed to be an important factor in the
evolution of life-history characteristics and habitat selection (Slagsvold 1982,
Sonerud 1985a, Martin 1988, Bosque and Bosque 1995). However, my data suggest
that nest predation was not associated with territory quality in Flammulated Owls. In
addition to the fact that nest success across territories was high (82% over the study),
as it is in most cavity-nesters compared with open nesting birds (Ricklefs 1969,
Wilcove 1985), I found no differences in predation rates among owl territories, and
no differences among forest types where nests were located. Density of Red Squirrel
middens, which I used as a measure of relative squirrel density in territories, was
highest in Douglas-fir forests where trees had highest density. This is consistent with
other studies that found denser stands of coniferous forests were preferred habitat for
Red Squirrels (Rusch and Reeder 1978, Gurnell 1984). However, density of middens
was not correlated with owl productivity, providing no evidence that Red Squirrels
affected habitat selection by owls. These results contrast with those of Korpimaki
(1993) and Sonerud (1985b), who found that rates of nest predation were high in
populations of cavity-nesting Tengmalm’s Owl (Aegolius funereus), and that owls
preferentially nested in areas where nest boxes had lowest predation rates.
Prey abundance affects the quality of breeding habitats for many birds
including raptors (e.g., Janes 1984, Korpimaki 1988, Burke and Nol 1998). However,
163
I found no evidence that arthropod abundance over the 2 yr sampling period differed
between a high-productivity and low-productivity territory. Moreover, annual
constancy in owl reproductive variables (Chapter 2) suggests that any stochasticity
associated with arthropod abundance during the breeding season has had little effect
on long-term productivity. Still, research on prey abundance is needed over more yr
to better assess the relationship between prey and productivity, in addition to two
other aspects of prey abundance. Because topographic temperature gradients often
become established after dark (pers. observ.), study is needed to determine if flying
arthropods follow warmer temperatures by migrating to higher slope positions, where
ponderosa pine/Douglas-fir forests typically occur. In addition, because energetic
demands by females peak during egg-formation (generally late May in Colorado),
when nights are cold and arthropod activity is low, more study is needed to determine
if arthropod abundance during this possible ‘bottleneck’ period is correlated with owl
productivity.
Positive correlations between productivity and ponderosa pine/Douglas-fir
forests likely reflect the importance of these forests and their structure in the
behavioral ecology of Flammulated Owls. I previously reported that male owls,
which provide almost all the food for their mates and owlets until fledging, foraged
significantly more often in forests of ponderosa pine/Douglas-fir than in other forest
types (Linkhart et al. 1998). This forest type may be important for males because the
characteristically large, open tree crowns facilitate their gleaning and hover-gleaning
foraging tactics within and on the surface of tree crowns (Linkhart et al. 1998,
Reynolds and Linkhart 1987). Thus, as long as arthropod density is not limiting,
164
relative arthropod abundance among forest types or among territories may not be as
important as vegetation structure that facilitates capture of arthropods. Structure
associated with forests of ponderosa pine/Douglas-fir is also likely important to other
behaviors; I reported previously that owls preferentially used older (larger) ponderosa
pine and Douglas-fir trees for singing and day-roosting, probably because these trees
provided more protective cover against inclement weather and predators than younger
trees (Linkhart et al. 1998).
Bases for Inferring Territory Quality
Many studies have inferred habitat quality based on relative abundance (see
sources cited in Introduction) or duration of occupancy (e.g., Moller 1983, Bunzel
and Druke 1989, Newton 1989, Matthysen 1990). In my study occupied yr, which
was equivalent to duration of occupancy by breeding pairs and bachelor males, was
correlated only with territory area in ponderosa pine/Douglas-fir forest and showed
no correlation with any forest structure variables. This was because bachelor males
that occupied up to 70% of territories annually (Chapter 2) were positively associated
with sub-optimal habitat conditions. Therefore, inferring habitat quality based on
density or occupancy alone may be unreliable if mating status or reproductive success
of individual males is not known, as has been previously suggested (e.g., Van Horne
1983, Robinson 1992).
Breeding yr, which was identical to total nesting attempts in this study, was
nearly equivalent to total owlets fledged in habitat correlations because (1) nesting
success was high (82%), since relatively few nests were lost to predators, (2) range
of clutch size was very small (clutches almost always contained either 2 or 3 eggs),
165
among the least of North American strigiforms (Chapter 2), and (3) most eggs
hatched and survived to fledging (Chapter 2). Consequently, breeding yr may be a
good surrogate for owlets in inferences of habitat quality in other portion of the
Flammulated Owl’s range, if breeding yr is associated with productivity as it was in
my study. Pairing status was used in assessments of habitat quality in other studies,
although the equivalency of this variable to productivity was not known (e.g., Burke
and Nol 1998).
Mean tenure on individual territories, which I used as an estimate of survival,
did not differ among territories for either sex suggesting that territory quality did not
influence individual survival. These data should be viewed cautiously for males,
however, since they were based primarily on males that nested more than once and
not on bachelor males (capturing bachelor males was difficult). Nonetheless, tenure
of males that were unpaired up to 5 yr between nesting attempts was similar to males
who nested annually (Chapter 4), suggesting that survival did not differ between
bachelor and breeding males. For males, this may indicate that survival is more
influenced outside of the breeding season, such as during migration, when many
Neotropical migrants species suffer mortality (Gill 1999). For females, survival may
be affected more by the energetic cost associated with both migration and egg-laying
(Chapter 2) than by selection of breeding territory. Tenure or turnover on territories
was influenced by territory quality in other studies of habitat quality (Marquiss and
Newton 1982, Korpimaki 1988). While the number of unique pair bonds on
territories ranged from 1 (A24, A18, and A2 territories) to 10 (A4 territory), the
frequency of this variable was primarily a function of the total number of breeding
166
pairs (= breeding yr) and was not a reflection of mate fidelity. Mean bond duration
did not differ among territories, likely because bond duration was limited by tenure of
females on territories (Chapter 4) rather than territory quality. Other studies have
found that mate fidelity differed among territories, suggesting that mate fidelity was
associated with habitat quality (Newton and Marquiss 1982, Haig and Oring 1988,
Part 1994).
The fact that most territories eventually produced owlets indicates that long-
term demographic data are needed to make accurate inferences regarding habitat
quality. While low-productivity territories were typically occupied by bachelors
annually, occasionally these territories were occupied by males that bred for 1-3
consecutive years. Short-term studies that captured only the years in which breeding
occurred on these territories would have identified relative territory quality much
differently than I have over the 19 yr study, resulting in inaccurate interpretations of
habitat quality. In addition, because most Flammulated Owls remained on territories
their entire known reproductive lives (up to 12 yr; Chapter 4), and because lifetime
reproduction was primarily a function of longevity (Chapter 3), long-term study was
required to assess contributions of individuals. Other studies have shown that long
time periods are necessary to evaluate effects of atypical individuals and extreme
environmental conditions (e.g., Woolfenden and Fitzpatrick 1984, Van Horne et al.
1997).
Stability of Territories
Considering the marked variation in reproductive success among territories,
frequent changes in territory boundaries and usurpation of territory ownership might
167
be expected if males attempted to improve their fitness (Janes 1984). However,
territory boundaries did not often change, possibly due to stable densities of cavity
trees, since other studies have found that territory boundaries shifted in response to
changing cavity resources (van Balen et al. 1982, East and Perrins 1988). But why
would males occupy low-quality territories their entire known reproductive lives
without dispersing to higher-quality territories? One possibility is that it is more
profitable for a male of a long-lived species such as the Flammulated Owl to await
the probable arrival of a female in a territory where he is familiar with the location of
sites for feeding, nesting, and escaping predators (Greenwood 1980). In addition,
males may "hedge their bets" by engaging in extra-pair copulations (EPCs; Reynolds
and Linkhart 1990, pers. observ.). Other raptor studies have found that territory
boundaries remained fixed or changed little year-to-year despite turnover of territory
occupants (Southern 1970, Newton 1976, Janes 1984, Nicholls and Fuller 1987), or
that males remained on the same territories even when higher-quality territories were
apparently available (e.g., Janes 1984, Woolfenden and Fitzpatrick 1984, Korpimaki
1988).
Distribution of Territories on the Landscape
Several authors reported finding clusters of singing male Flammulated Owls
separated by relatively large areas of apparently unoccupied habitat (e.g., Marcot and
Hill 1980, Howie and Ritcey 1987), a phenomenon that led some previous researchers
to postulate that the owls may be semi-colonial (see Winter 1974). Such clusters do
not provide direct evidence for coloniality, because observations are not based on
locations of nests but rather on responsiveness of singing males, which may or may
168
not be nesting. Compared to nesting males, unpaired males are more likely to be
detected because they sing with greater duration through the night and through the
breeding season (Linkhart et al. 1998). Aggregations of nesting territories, where
they exist, may reflect either surrounding areas of suitable habitat that are unoccupied
(Winter 1974), or surrounding habitat that appears suitable for breeding, but is in fact
suboptimal (Howie and Ritcey 1987, Reynolds and Linkhart 1992). My data support
the latter hypothesis, because male owls in this study only established territories
where suitable cavity trees for nesting were available, leaving unoccupied some
surrounding areas of habitat that appeared potentially suitable for breeding (i.e.,
forests consisting of mature, relatively open ponderosa pine/Douglas-fir), but lacked
cavity trees. Although density of cavity trees was not correlated with reproductive
success, territories were generally aggregated around clusters of cavities, which
occurred primarily in large quaking aspen trees in bottom areas, and secondarily in
large conifers on ridge tops (Reynolds et al. 1985, pers. observ.).
Forest Management
The correlation of productivity on territories with higher densities of larger-
diameter trees suggests that Flammulated Owls are adapted to forests that were
historically maintained by fire. Fire supression in many western forests, which were
characterized by open stands of large-diameter trees prior to European settlement, has
resulted in higher tree densities especially in the smaller diameter classes and has
resulted in conversion of many pine forests to fir forests (Cooper 1960, Covington
and Moore 1992). Tree density in smaller diameter classes was negatively correlated
with productivity on territories, suggesting that fire suppression may be resulting in
169
sub-optimal habitat for Flammulated Owls. Research is needed to determine the
effects on owl productivity of prescribed burns and/or selective logging that return
forest structure to pre-settlement conditions.
Further research is required to determine how and if patterns evident on this
511 ha study area are applicable elsewhere. Generally, breeding Flammulated Owls
have been associated with mature montane forests throughout their range (McCallum
1994a). However, Flammulated Owls may not be restricted to breeding in forests of
ponderosa pine/Douglas-fir, as indicated by studies that found owls breed in nest
boxes at relatively high densities in quaking aspen stands of northern Utah (Marti
1997), and that owls breed in pure forests of Douglas-fir in Montana (Powers et al.
1996). In order to determine the importance of floristics and forest structure, and to
assess the effects of forest management activities on breeding Flammulated Owls
over their range, researchers need to undertake comparative demographic studies of
owls in different forest types and across multiple forest management regimes.
170
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CHAPTER VI
CONCLUSIONS
Several demographic characteristics indicated that Flammulated Owls have a
life history more typical of larger birds, which generally have lower fecundity and
longer breeding lifespans, higher nesting success, and are longer-lived than smaller
birds (Newton 1998, Ricklefs 2000). First, fecundity of Flammulated Owls is among
the lowest and least variable of North American and European strigiforms, despite the
fact that they are one of the smallest (Johnsgard 1988). Second, male Flammulated
Owls had longer breeding lifespans compared to other species of strigiforms for
which comparable data exist (Korpimaki 1992, Marti 1997). Third, at least 75% of
nests were successful in 16 of 19 years for an overall nesting success rate of 82% over
the study. Among North American strigiforms, only Spotted Owls (S. occidentalis)
have a higher reported nesting success (85%; Forsman et al. 1984). Fourth, I never
observed replacement clutches or multiple broods in Flammulated Owls. Among
raptors in temperate regions, several small species and some larger species are known
to lay a replacement clutch if the first clutch is lost at an early stage (Newton 1979,
Johnsgard 1988). Finally, male Flammulated Owls have maximum longevity (at least
12 yr) greater than other small (< 150 g) owls and longevity comparable to many
larger owls (Glutz and Bauer 1980, Clapp et al. 1983, Klimkiewicz and Futcher 1989,
Klimkiewicz, pers. comm).
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Data on lifetime reproductive success (LRS) indicated that a small percentage
of adult Flammulated Owls accounted for the majority of the total offspring produced
in the population, which is consistent with LRS data from other avian species
(Clutton-Brock 1988, Newton 1989, Wiklund 1995). Seventeen percent of females
and 27% of males accounted for 50% of all Flammulated Owl offspring, while in
other species the percentages for females range from 15% (Red-billed Gulls; Mills
1989) to 31% (Kingfishers; Bunzel and Druke 1989), and the percentages for males
range from 14% (Indigo Buntings; Payne 1989) to 30% in (Kingfishers; Bunzel and
Druke 1989). Males had longer breeding lifespans and more mates over their
lifetimes than females. These differences may result from males having greater
longevity than females (see below). Total breeding years were strongly correlated
with lifetime productivity for females and males, because clutch sizes varied little and
nesting success was high. Among all bird species, breeding lifespan has emerged as
the major demographic determinant of LRS (Newton 1989).
Male Flammulated Owls had significantly longer tenure on territories than
females, probably because males had greater territory fidelity (98% vs 78%) and an
apparently longer lifespan. Male-biased site fidelity is widespread among raptors
(e.g., Newton and Marquiss 1982, Forero et al. 1999), because in resource-defense
mating systems (Emlen and Oring 1977) males may have more to gain by being
faithful to breeding territories than females (Greenwood 1980). While longer lifespan
in male Flammulated Owls has not been substantiated, it is plausible for two reasons.
First, mean longevity over 19 yr was significantly greater for males than females,
although this disparity may in part reflect female-biased dispersal. Second, unpaired
182
males annually occupied 10-70% of territories, which suggests a shortage of females
in the breeding population. The apparent lack of females may be tied to energetics.
Flammulated Owls lay clutches representing a greater proportion of their mass (55-
60%) than other North American strigiforms (Johnsgard 1988). Coupled with the
high energetic cost of long-distance migration immediately prior to egg-laying, these
data suggest that females may be predisposed to higher mortality rates than males.
Female Flammulated Owls had a higher rate of breeding dispersal than males
(0.22 vs 0.02), a pattern that is well-documented in other birds (e.g., Greenwood
1980, Marquiss and Newton 1982, Forero et al. 1999). Dispersing females moved to
territories where productivity over the 19 yr study was significantly greater than on
territories from which they dispersed, suggesting that females were capable of
distinguishing relative quality among territories.
Fidelity and dispersal data suggest that dispersal distance by female owls may
be bimodal with females moving to adjacent territories following successful nests and
more distant territories (> 2 km) following unsuccessful nests. Dispersal by females
to nearby territories, as was found in many other species (e.g., Payne and Payne 1996,
Hannon and Martin 1996, Williams 1996, Forero et al. 1999), may be beneficial
because dispersers can best judge the quality of resources and potential mates in
adjacent territories (Hinde 1956, Greenwood 1980, Ens et al. 1996). Although
dispersal following nesting failure is beneficial if chances of future nesting success
are improved (Murphy 1996), benefits of dispersing to more distant territories, where
owls are unlikely to have knowledge of resources or potential mates, are not clear.
183
Most male Flammulated Owls occupied a single territory their entire known
reproductive lives. Whether or not males were paired annually did not appear to limit
their tenure or likelihood of changing territories. Males often continued to occupy
original territories despite being unpaired up to four consecutive years, even when
more productive territories apparently were available. Thus, high site fidelity by
males appeared to counter predictions based on habitat selection models that assume
animals select habitats conferring highest reproductive success, and that if higher-
quality habitats become available individuals should move to the new sites (Fretwell
and Lucas 1969). Consequently, territory fidelity by Flammulated Owls may be
considered a suboptimal form of habitat selection with respect to territory quality
(Switzer 1993). Males of other species including raptors have been documented
remaining on a single territory, even when higher-quality territories apparently were
available (e.g., Janes 1984, Woolfenden and Fitzpatrick 1984, Korpimaki 1988).
Productivity of owls over the 19 yr study was positively correlated with
territory area in ponderosa pine/Douglas-fir forests, and with greater crown volume in
the second-largest of four tree-diameter categories. Productivity was not correlated
with density of cavity-trees. However, cavity trees clearly distinguished territory
from non-territory habitat, since non-territory habitat contained < 10% of mean
cavity-tree density within territories. Few structural characteristics distinguished
combined territories from non-territory habitat. In comparisons of non-territory
habitat with three classes of territory distinguished by differing productivity, only the
high-productivity class contained greater tree density and basal area in the larger dbh
categories. Based on the fact that moderate-productivity territories showed no
184
differences in forest structure from unoccupied habitat, and that low-productivity
territories (usually occupied by bachelor males) actually contained denser forests and
smaller trees than non-territory habitat, at least some portions of non-territory habitat
may have been suitable for establishment of territories except for scarcity of cavity
trees. Monitoring of predation on artificial nests for 1 yr and relative prey abundance
for 2 yr revealed no patterns among selected territories differing in productivity,
suggesting these factors were not associated with habitat quality.
My results indicate that habitat quality was determined by two primary
factors. First, cavity-tree availability determined where owls established territories,
and second, forest type and structure determined whether a territory was more often
occupied by breeding pairs or by bachelor males. High-quality breeding habitat for
Flammulated Owls in this study was characterized as mature, relatively open stands
of ponderosa pine/Douglas-fir that contained sufficient cavity trees for nesting.
Many studies have inferred habitat quality based on relative abundance or
duration of occupancy (e.g., Bunzel and Druke 1989, Newton 1989). In my study
occupied yr, which was equivalent to duration of occupancy by breeding pairs and
bachelor males, was correlated only with territory area in ponderosa pine/Douglas-fir
forest and showed no correlation with any forest structure variables. This was
because bachelor males that occupied up to 70% of territories annually were
positively associated with sub-optimal habitat conditions. Therefore, inferring habitat
quality based on density or occupancy alone may be unreliable or misleading if
mating status or reproductive success of individual males is not known, as has been
previously suggested (e.g., Van Horne 1983, Robinson 1992). Breeding yr may be a
185
good surrogate for productivity in future efforts to identify important breeding
habitats for this species, at least where other demographic parameters such as nesting
success are similar to mine.
Habitat correlations in this study suggest that environmental changes,
including fire suppression, may affect long-term viability of owl populations. The
open structure and species composition of ponderosa pine/Douglas-fir forests
throughout the western United States were historically maintained by frequent, low-
intensity ground fires (Cooper 1960). Fire suppression has resulted in increased tree
densities in ponderosa pine and mixed-conifer forests, has converted many pine
forests to fir forests, and has changed fire type from low-intensity to catastrophic,
habitat-destroying, crown fire (Barrett et al. 1980, Gordon 1980). In this study,
younger, dense stands of Douglas-fir were associated with sub-optimal breeding
habitat. In addition, many ponderosa pine and mixed-conifer forests within the range
of the Flammulated Owl have been harvested. These habitat changes have resulted in
declines of Flammulated Owls in some areas (e.g., Marshall 1957, 1988; Franzreb
and Ohmart 1978). Species exhibiting characteristics of K-selection (sensu Pianka
1970), such as Flammulated Owls, generally can be expected to respond slowly to
environmental perturbations because of their low fecundity and low density (Newton
1998). In order to understand effects of changes on structure and species composition
of owl breeding habitat, and on long-term impact to the reproduction and survival of
owl populations, researchers need to undertake comparative demographic studies of
owls across multiple forest management regimes and forest types.
186
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