Bats Bridges 2

Copyright ©1999 Bat Conservation International, Inc.

Permission is granted to interested persons to reproduce any page of this book provided Bat ConservationInternational is cited. Any use or reproduction of this document or any material contained within it mustbe used for conservation information or educational purposes. For all other purposes, permission must berequested in writing from Bat Conservation International, Inc.

All rights reserved by Bat Conservation International, Inc., P.O. Box 162603, Austin, TX 78716.

(512) 327-9721 Fax: (512) 327-9724

www.batcon.org

BATSIN

AMERICAN BRIDGES

BRIAN W. KEELEY

MERLIN D. TUTTLE

Bat ConservationInternational, Inc.

RESOURCE PUBLICATION NO. 4

Bridges and culverts were evaluated as bat roosting habitatin 25 states at elevations from sea level to 10,000 feet. Field

surveys were conducted at 2,421 highway structures. Scientificliterature was reviewed, and local biologists and engineers wereinterviewed, leading to the discovery of approximately 4,250,000bats of 24 species living in 211 highway structures. Only onepercent of existing structures had ideal conditions for dayroosting, but at little or no extra cost a much larger percentagecould provide habitat for bats in the future. Most species choseconcrete crevices that were sealed at the top, at least 6 to 12 inchesdeep, 0.5 to 1.25 inches wide, and 10 feet or more above ground,typically not located over busy roadways. Retrofitting existingbridges and culverts proved highly successful in attracting bats,especially where bats were already using them at night.

Providing bat habitat in bridges or culverts, either duringinitial construction or through subsequent retrofitting, is anexceptionally feasible and popular means of mitigation that ishighly cost-effective in demonstrating a proactive commitment tothe environment. Advice for incorporating bat roosts, both beforeand after construction, is provided. Environmental and economicbenefits, impacts on structural integrity and public safety, andmanagement of occupied structures are discussed.

This document is available to the public online at www.batcon.org.

Summary

Cover photo by Merlin D. TuttleBack cover photo by Karen Marks

Federal Highways AdministrationTexas Department of Transportation

Florida Department of TransportationGeorgia Department of Transportation

Oklahoma Department of TransportationOklahoma Department of Wildlife

ConservationTennessee Department of TransportationWyoming Department of Transportation

New Mexico State Highway and TransportationDepartment

Utah Department of TransportationArkansas Department of Transportation

J.D. Abrams Inc.U.S. Corps of Engineers

Bureau of Land ManagementOregon Department of Fish and WildlifeMargaret Cullinan Wray Charitable Trust

National Fish and Wildlife FoundationThe Winslow Foundation

Acknowledgements

Support from the North American Bat Conservation Partnership andfinancial assistance from the following organizations made this project

possible. Their contributions demonstrate foresight and understanding inbalancing human activities with a healthy environment.

We especially thank the Federal Highways Administration and the TexasDepartment of Transportation for their roles as lead agencies in initiatingthis project.

Bat ConservationInternational

U.S. Depart m e n tof Tr a n s p o rt a t i o nFederal HighwayA d m i n i s t r a t i o n

Lloyd Wolf, TXDOTMark Wallace, TXDOTDavid Zeigler, FLDOTSteve Friedman, GADOTSteve Wiedl, GADOTPaul Lyles, GADOTKyle McKinley, OKDOTRaymond Brisson, TNDOTKeven Brown, TNDOTMike Presley, TNDOTDr. Sam Musser, UTDOTLaura Romin, UTDOTMary Ann McGraw, NMDOTCraig Conley, NMDOTRob Doster, ARDOTCharlie Rountree, IDDOTLui Chen, LADOTTim Stark, WYDOTDebra Ferguson, WYDOTJody Sawasaki, NVDOTDebra Starnes, NVDOTDon West, VADOTVincent Pizzolato, LADOTEd Frierson, SCDOTClaiborne Barnwell, MSDOTGene Gardner, MODOTGeorge Rose, ILDOTBrian Moyers, KYDOTGregg Erickson, CALTRANSBill Knight, AZDOTAnnika Keeley, volunteer

Andy Moore, BCISteve Walker, BCIAngela England, BCIMark and Selena Kiser, BCIJoan Ivy, BCILen Carlton, volunteerJoan Childs, volunteerBrian Knoll, volunteerBrendan and Gayle Keeley,

volunteersJeff and Hope Camper,

volunteersGene and Nancy CushionKathy Latendress, volunteerBob Currie, USFWSDavid Dell, USFWSRick Reynolds, VAG&IFJeff Gore FLG&FJim Ozier, GAF&GBetsy Bolster, CAF&GKeith Hutson, ALG&FAndy Holycross, AZG&FGlenn Fredericks, AZG&FKirk Navo, COG&FBob Luce, WYG&FRandy Sims, NTNPKathy Jo Wall, BLMTheresa Bolch, BLMSteve Langenstein, BLMDan Van Dyke, ODF&WDave Clayton, USFS

Kerensa King, USFSDavid Saugey, USFSPat Ormsbee, USFSRussell Orr, USFSJohn MacGregor, USFSDave Kennedy, USFSRichard Lance, LSURick Sherwin, BYUDr. Steve Cross, SOUDale Sparks, FHSUDr. Scott Altenbach, NMSUDr. Gary McCracken, UTKDr. Thomas Kunz, BUDr. Jerry Choate, FHSUDr. Paul Racey, UA, EnglandDr. Elizabeth D. Pierson, UCBDr. Patricia Brown and Bob

Berry, consultantsJo McDonnell, NCUColin Cato, BCTGlenn HoyeThomas Barnard, LoBuono,

Armstrong & AssociatesHill Henry, TVAAmy Goebel, consultantLaura Finn, consultantJohn Storrer, consultantMatt Anderson, TNCPhillip McKenna, MARTASam Guy, MARTA

Our appreciation goes to Dr. Paul Garrett, of the Federal Highways Administra-tion and Mark Bloschock, P.E., from the Texas Department of Transportation, fortheir vital leadership. Thanks also to Jeannette Ivy for layout and editing adviceand Gary Evink with the Florida Department of Transportation for his review.

The following individuals contributed a wide variety of information, advice, andfield assistance.

Importance and Needs of Bats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Benefits of Bats and Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

National Survey Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Characteristics Bats Prefer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Bats and Highway Structure Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

How to Evaluate Highway Structures for Bat Use . . . . . . . . . . . . . . . . . . . . . . . . .17

How to Create Bat Roosts in Highway Structures . . . . . . . . . . . . . . . . . . . . . . . . .18

Mitigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

Commonly Asked Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

Appendices

I. Bats that Use Bridges and Culverts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

II. Survey Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

III. Retrofitting for Bats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

IV. Useful References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38

V. State Survey Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40

VI. Ecoregions of the United States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41

Table of Contents

8 BATS IN AMERICAN BRIDGES

Bat Conservation International, Inc. www.batcon.org

The Importance and Needs of Bats

BATS ARE an indispensable natural resource.As primary predators of insects that fly at night,

they are essential to the balance of nature. Bats alsoconsume enormous quantities of insect pests thatcost farmers and foresters billions of dollars annually.

Mexican free-tailed bats (Tadarida brasiliensis),common bridge-dwellers across the southern U.S.,intercept northward migrations of America’s mostcostly agricultural pest, the corn earworm moth(Figure 1). In Texas, these bats consume an estimated2 million pounds of insects each night, reducingmigrations of these and other pests that affect farmersthroughout the U.S. farm-belt and as far north asCanada (McCracken, 1996) (Figure 2).

One bat can easily eat 20 female corn earwormmoths in a night, and each moth can lay as many as500 eggs, potentially producing 10,000 crop-damagingcaterpillars. Yet as few as eight caterpillars per 100plants can force a farmer to apply pesticides, demon-strating the impact of even small bat colonies.

Unfortunately, more than half of America’s bats,even species traditionally viewed as common, areendangered or declining in numbers sufficient towarrant concern. Bats are especially susceptible toextinction because most species form large colonies

Figure 1. Mexican free-tailed bat eating a corn earwormmoth (Helicoverpa zea).

Figure 2. Dopplerradar images reveallarge bat emergencesspreading to engulfconcentrations ofinsects rising forcrop lands where

corn earworm mothsare primary pests.

Contemporary engineers are students “of the engineering sciences”: materials, structures, fluids, electricity, light,heat, energy, chemicals, systems—all the phenomena that constitute the physical universe. Underlying these engineeringsciences are three fundamental disciplines: mathematics, physics, and chemistry—and increasingly, a fourth: biology.

—Samuel Florman, The Introspective Engineer

in vulnerable locations, such as caves, and produceonly one pup per year. Colonies numbering in themillions have died when their roosts were disturbedor destroyed. As a consequence of losing naturalroosts in caves and old growth forest snags, bridgesand culverts have become havens of last resort.

Insects

Bats

BATS IN AMERICAN BRIDGES 9


The Benefits of Bats and Bridges

TWENTY-FOUR of the 45 U.S. species of bats havebeen documented to use highway structures as

day and night roosts, and based on their knownpreferences at least 13 others are likely to do so.Although only one percent of American highwaystructures provide ideal day roost conditions, minormodifications in the design of future structures couldeasily provide homes for millions of bats.

Transportation departments can incorporate batroosting spaces as a key element of on-site mitigation,to demonstrate proactive commitments to the envi-ronment, aid farmers, and gain positive publicity,often at little or no extra cost to the taxpayer(Figure 3).

The Congress Avenue bridge in Austin, Texas,clearly illustrates the economic and ecological benefitsof bats in bridges (front cover). In the early 1980s,modifications to the bridge began attracting Mexicanfree-tailed bats. Citizens panicked when local mediaportrayed bats as dangerous (Figure 4). But fearquickly turned into appreciation as Austinites learnedthat the estimated 1.5 million bats consume 10 to 15tons of insects each night, and their spectacular emer-gences attract tourist dollars. The Texas Departmentof Transportation, Bat Conservation International,and the City of Austin designed and installed view-ing areas and educational kiosks, as well as bilingualsigns warning people not to handle grounded bats.Now the citizens have proudly dubbed Austin the“Bat Capital of America,” and the bridge is listed as

a top tourist attraction on the City of Austin Webpage. Each year, it attracts tens of thousands oftourists from all over the world.

The Bats in American Bridges Project compiledthis report to help transportation departments pro-vide bat habitat where appropriate while avoidingit where nuisances could result. The report describesnationwide survey results for bat use of highway

structures, preferred roost characteris-tics, roost enhancement techniques,and information on how state trans-portation departments are handlingbat-related issues.

Figure 3.Many of America’s leading magazines,newspapers, and television shows have covered the batsand bridges story.

Figure 4. Sensational mediastories caused panic byportraying the bats in Austin’sCongress Avenue bridge asdangerous animals.



FIELD STUDIES, literature reviews, and interviewswith biologists and engineers were conducted to

determine which bat species use American highwaystructures, to identify their roosting preferences, andto develop methods of predicting where bats will usethem.

A total of 2,421 structures (1,312 bridges and1,109 culverts) were surveyed for bat use along aroute that passed through 25 states primarily in thesouthern half of the U.S. (Figure 5). Sixty differentcharacteristics were used to determine bat roostingpreferences. Sample survey forms are available inAppendix II.

Field surveys were impractical for bridges andculverts in the remaining 23 northern states becausefew are warm enough to meet bat needs. For thesestates we relied only on interviews. Hawaii has nobats likely to use highway structures.

National Survey Design

Figure 6. This Idaho bridge sheltered a nursery colonyof several hundred little brown myotis, the northernmostcolony discovered during the survey.

Figure 5. National survey results.

2,421 structures surveyed211 occupied as day roosts25 southern states with field surveys23 northern states with interviews only

Symbols are overlain forstructures that are close

to each other.



Characteristics Bats Prefer

BATS USE HIGHWAY STRUCTURES either as dayor night roosts. Day roosts are places that protect

bats from predators and buffer weather changeswhile resting or rearing their young. Such roosts are

Figure 8. Night roosts are usually located in open spacesbetween bridge beams.

Figure 7. Bridge crevices provide ideal day roost conditionsfor a fringe-tailed myotis (upper) and these Mexican free-tailed bats (lower).

usually in expansion joints or other crevices (Figure7). In contrast, night roosts, where bats gather todigest their food between nightly feeding bouts, areoften found in open areas between bridge supportbeams that are protected from the wind(Figure 8).

Two hundred and eleven highway structureswere used as day roosts and 94 percent were occu-pied by crevice-dwelling bat species. Seven hundredand fourteen highway structures were used as nightroosts. Day and night roost survey totals are listed inAppendix V.



Day Roosts

Only 281 of the 2,421 structures surveyed hadcharacteristics that met the minimum needs ofday-roosting bats. Ideal day roost characteristicsfor crevice-dwelling bat species that use highwaystructures, include (in descending priority):

Bridges (Figure 9):

• location in relatively warm areas, primarily insouthern half of the U.S.

• construction material: concrete• vertical crevices: 0.5 to 1.25 inches (0.25 to

3 centimeters) wide• vertical crevices 12 inches (30 centimeters)

or greater in depth• roost height: 10 feet (3 meters) or more above

the ground• rainwater-sealed at the top• full sun exposure of the structure• not situated over busy roadways

Culverts:

• location in relatively warm areas• concrete box culverts• between 5 and 10 feet (1.5 and 3 meters) tall

and 300 feet (100 meters) or more long• openings protected from high winds• not susceptible to flooding• inner areas relatively dark with roughened walls

or ceilings• crevices, imperfections, or swallow nests (Figure 10)

Figure 10. Cave myotis were found using swallow nestsin concrete box culverts.

Figure 9.

Water-tight seal

Full sun for amajority ofthe day

Crevices0.75 to 1-inches wide

(1.9 to 2.5 cm)10 feet

(3 m) or more abovethe ground

Not situated overbusy roadways

Crevices12 inches(30 cm)

or greaterin depth

Concrete asprimary

constructionmaterial

Located inthe southern

half of theU.S.



Bats use parallel box beam bridges as day roostsmore than any other kind (Figure 27, page 31). Thenext most preferred bridges are cast in place or madeof prestressed concrete girder spans. These designsare the most likely to contain spaces suitable for bats.Although parallel box beam bridges were rarelyencountered during the survey, they can providenumerous crevices of suitable width. Metal and smallconcrete culverts are the most frequently encounteredhighway structures and are the least preferred asroosts.

We found substantial variation in the frequencywith which bats used suitable highway structures aseither day or night roosts. Even ideal structures wererarely used by bats in areas dominated by openplains, perhaps due to a lack of appropriate habitat.Figure 12 compares average use rates with habitattypes according to the major ecoregions defined byBailey, 1995 (Appendix VI).

Many of the day-roosts were found in opencrevices exposed to weather and predation, makingthem highly vulnerable to disturbance and injury byhumans or vehicles (Figure 11). Although concrete isthe preferred roost material, bats sometimes usedwooden roosts or, when desperate, metal.

Figure 11. Desperate for roosts, this nurserycolony of Alabama big brown bats (Eptesicusfuscus) roosts in an open crevice, exposed todisturbance from traffic and the weather.

1,449 suitable structures*No field surveys75 to 100% occupancy50 to 75% occupancy25 to 50% occupancy0 to 25% occupancy

* Allsuitablestructuresinclude typesused as day ornight roosts. Flatspan, steel culverts,or small concreteculverts are not included.

Figure 12. Highway structure use patterns by ecoregion(See Appendix VI)



Figure 13. An estimated 35,000 cave myotisraise their young in this south Texas culvert.

Figure 14. Lei Jin of the Louisiana Departmentof Transportation and Brian Keeley examine aRafinesque’s big-eared bat colony roosting in theopen beams of a bridge.

Night RoostsBats frequently use highway structures as nightroosts. In fact, 29 percent of all structures surveyedhad signs of night-roost activity. In some regions ofthe southwest, all suitable structures were used bynight-roosting bats.

Night-roosting bats are believed to be attractedto bridges that provide protected roosts and havea large thermal mass that remains warm at night.Bridges constructed of prestressed concrete girderspans, cast-in-place spans, or steel I-beams are pre-ferred. Vertical concrete surfaces located betweenbeams provide ideal protection from wind and areespecially used when they are heated by full sunexposure. Bats typically do not use bridges with flatbottomed surfaces that lack inter-beam spaces. Theywill avoid small culverts but will roost at night in thelong concrete box culverts that often pass underdivided highways, if the culverts are at least 5 feet(1.5 meters) tall.

Bats use night roosts in bridges mostly between10 p.m. and midnight. Some remain for most of thenight, periodically feeding and returning to digesttheir meals. Night roosts appear to play importantroles in body temperature regulation and socialbehavior.

Species PreferencesSeventeen of the twenty-four species reported to usebridges or culverts were encountered during the fieldsurveys (Figure 16). Occupied day roosts ranged insize from a single male to nursery colonies with morethan one million mothers and their pups. Bridges andculverts are used by both bachelor and nurserycolonies, and as temporary roosts during migrationand mating. Culverts were sometimes also used forhibernation in southern areas.

Mexican free-tailed bat (Tadarida brasiliensis)colonies were found in southern bridges and culvertsfrom coast to coast. Although most colonies are com-posed of fewer than 100 individuals, Mexicanfree-tailed bats have the potential to form bridgecolonies numbering in the millions. The largestcolonies exist in Texas, New Mexico, Arizona, andCalifornia.

Big brown bats (Eptesicus fuscus) were thesecond most abundant bridge-dwellers. This speciesrepresented 21.5 percent of the day-roosting coloniesencountered. This species was found throughout theU.S. in small colonies ranging from two to seventyindividuals.

Cave myotis (Myotis velifer) colonies represented19 percent of the roosts encountered. Most were smallwith two to 10 individuals, but one nursery colony ina south Texas culvert included approximately 35,000


individuals (Figure 13). Abandoned swallow nestswere regularly used.

The evening bat (Nyctceius humeralis) and mostof the remaining myotis species were typically foundin colonies of 2 to 200 individuals in bridge crevices,although some colonies consisted of more than 1,000.Southeastern myotis (Myotis austroriparius) use bothbridges and culverts as nursery roosts, sometimeswith as many as 2,000 to 3,000 mothers and theirpups.

Unlike other bridge-dwelling species, botheastern and western pipistrelles (Pipistrellus subflavusand hesperus) and both Townsend’s and Rafinesque’sbig-eared bats (Corynorhinus townsendii andrafinesquii) were found roosting in the open betweenbridge beams (Figures 14 and 15). Rafinesque’sbig-eared bats were found rearing young betweenopen beams in low bridges darkened by thickvegetation bordering the sides. In one case a colonyof big-eared bats abandoned its roost immediatelyafter vegetation was removed. They returned threeyears later, when it had regrown (J. MacGregor, pers.comm.). In the southwest, individual maleTownsend’s big-eared bats were occasionally foundroosting in 5-foot diameter (1.5 meters) or largercorrugated metal culverts. The nectar-feeding Mexi-can long-tongued bat (Choeronycterus mexicanus) hasbeen reported using small diameter corrugated metalculverts (18 to 24 inches/45 to 61 centimeters) asday roosts in Arizona. Evidence of night roostingby small groups of nectar feeding bats was found inArizona bridges.

Maternity colonies of both the endangered graymyotis (Myotis grisescens) and Indianamyotis (Myotis sodalis) live in bridges.Hundreds to thousands of graymyotis were found rearing theiryoung in long concrete box cul-verts in three states.

The most frequent nightroost signs encounteredappeared to be from thegenus Myotis. Similar signsfrom big brown and big-eared bats were alsocommon regionally.Although Mexican free-tailed bats seemed to preferto use their roost crevices asboth day and night roosts, theywere sometimes found nightroosting in large numbersbetween open bridge beams and inlong, tall concrete box culverts.

Figure 15. Unlike crevice-dwelling bats, pipistrelles willday roost between open beams.

Figure 16. Species in Occupied Structures

California myotis0.3%

Western small-footed myotis0.3%

Western pipistrelle0.3%

Fringed myotis0.7%

Long-eared myotis1.0%

Eastern pipistrelle1.0%

Townsend’s big-eared bat2.8%

Yuma myotis2.8%

Pallid bat2.4%

Evening bat2.1%

Gray myotis1.7%

Unknown1.0%

Little brown myotis4.8%

Southeastern myotis3.5%

Rafinesque’s big-eared bat3.1%

Mexican free-tailed bat

26.0%

Big brown bat21.5%

Myotis species5.5%

Cave myotis19.0%




BATS HAVE THE LARGEST SURFACE AREAto body mass of any mammal, and this requires

greater energy to maintain body temperatures.Sun-warmed bridges help adult bats to conserveenergy and foster development of their young.

During the summer months, sun-exposed bridgesact as thermal sinks, often achieving and holdingtemperatures above the ambient average for most ofthe 24-hour cycle (Figure 17). Comparisons of ambi-ent and bridge temperatures from roosts in Kentucky,Texas, Oregon, and California show a similar pattern(J. MacGregor and D. Clayton, pers. comm.). Thehigher, more consistent bridge temperatures areespecially important in mountainous or desertregions where ambient temperatures fluctuatedramatically within a 24-hour cycle.

An Oregon study found that bats prefer bridgeswith greatest sun exposures. Bridges receiving nosun had little or no bat use. This preference wasespecially obvious within partially shaded bridges,where roosting activities occurred only in thesun-exposed halves of bridges (Keeley, 1998).

Bats and Highway Structure TemperaturesThe northernmost day roost discovered in this

study was occupied by a maternity colony of roughly300 little brown myotis in an Idaho bridge at 44°north latitude. In the eastern U.S. we found occupiedbridges as far north as Virginia and Kentucky andhave reports of occupied bridges from Indiana andNew Jersey. However, the number of day roostsappears to drop rapidly above 42° north latitude.

Figure 17. Changes in Bridge Temperature over 24 Hours

1:30 PM 8:30 PM 1:30 AM 6:30 AM

95

90

85

80

75

70

Time of Day

Average temperatures taken simultaneously from 4 occupied bridges in central Texas Ambienttemperature

Bridgetemperature



How to Evaluate Highway Stru c t u res for Bat Use

CONDUCTING SURVEYS using appropriatetechniques can provide valuable information on

where bats use highway structures and what charac-teristics they prefer. Evaluating bridges or culverts forbats or signs of bat use is easy if you know what tolook for. Knowing where highway structures are usedby bats alerts planners to areas where enhancement,mitigation, or exclusion projects may be most needed.

Conducting SurveysThe type of bat roosting information needed willdetermine the survey design. Educating bridgeinspectors to recognize evidence of bat use can easilyprovide information on the distribution of bat-occu-pied structures. More detailed information oridentification of species and their roost preferencescan be obtained by trained biologists.

The Florida Department of Transportation asksits district bridge inspectors to include notes on batroosts in their reports, which are periodically com-piled by the environmental planning department.This type of survey adds little effort to existingworkloads and inexpensively produces a statewidedatabase of bat-occupied bridges in a relatively shortperiod of time. The information is useful for planningstructural maintenance schedules and predicting batoccupancy for mitigation projects.

Parallel box beam bridges with suitable crevicesare the most frequently used highway structuredesign for day roosts. Lists of parallel box beambridge locations are available from the bridge divisionof transportation departments. Surveying thesebridges for bat use can provide quick assessment.

To more fully evaluate bat use patterns, a statecan be divided into sections, either by districts or bygeographic or ecological regions. Intersections can beselected along all major road types (interstates, U.S.highways, state highways, and county roads) withineach section. From each intersection, equal distancesare traveled down each roadway, surveying at least30 structures. Using the data sheets provided inAppendix II, all information for each structure isrecorded for later comparison. If needed, additionalinformation about structures can be obtained fromthe Bridge Inspection and Appraisal Program(BRINSAP) files maintained by each statetransportation department.

Survey TechniquesEvidence of use in occupied day roosts often includesvisible bats, audible chirping, as well as droppings

and stains from urine or body oils at or below theroost. Special equipment can be helpful in locatingbat roosts and for identifying bats in unreachablelocations. A high-powered rechargeable light (500,000candlepower or greater recommended) combinedwith a pair of binoculars is useful for visual inspectionof dark crevices or cavities from beneath the bridgeor when looking inside culverts. Bridge crevices onroadways with low traffic loads can sometimes beinspected topside with a high-powered light. Anelectronic device called a bat detector can also beused to listen for high frequency vocalizations thatwould otherwise be inaudible to humans. A mirror orminiature camera lens mounted on a telescoping polecan aid inspections of otherwise unreachable locations(Figure 18).

Because suitable day roost conditions are rarein current highway structures, evidence left bynight- roosting bats is the most reliable method fordetermining activity in the area. Night roost signsare usually found under the bridge on or below thewarmest locations, such as between bridge beams,on vertical concrete surfaces, at the highest points(close to the road deck), and usually near the endabutments where the airflow is reduced. Appendix IIprovides survey forms useful for identifying preferredroosting characteristics in bridges and culverts.

Figure 18.To surveyunreachableroosts,researchersuse a cameralens attachedto a telescopingpole.



How to Create Bat Roosts in Highway Structures

CREATION OF DAY-ROOST habitat for batsin new or existing highway structures is easy,

often at little or no extra cost to the taxpayer. Fornew structures, the minimum needs for day-roostingbats can be met by specifying the proper dimensionsfor crevices such as expansion joints (Figure 19).Retrofitting habitat into existing highway structureshas become a popular and successful method ofaccommodating bats. Design plans for retrofittingbridges and culverts are available in Appendix III.Pre-surveys to look for bat signs in nearby bridgesare useful to predict the success of proposed enhance-ment projects. Four bridges in Oregon (D. Clayton,pers. comm.) and five bridges and two culverts inTexas with signs of night roosting were retrofittedwith ideal crevices, and all were occupied by batswithin the first year. All retrofit designs tested inbridges and culverts so far have successfully attractedbats, and at least six states are already usingretrofitting projects to accommodate bats.

Retrofitting projects have many appealingfeatures for habitat enhancement. They• are adaptable to almost any structure• can be placed where they will have a high potential

for success• can be placed in locations that minimize distur-

bance from maintenance or vandalism• can be sized to accommodate small or large colonies• are beneficial to agriculture

•are inexpensive (can be constructed from recycledmaterials)

• can be expanded by adding additional units ifinitial efforts are successful

• can be easily moved if necessary

Two basic designs can be used to retrofit almostany bridge or culvert. Texas Bat-Abodes (AppendixIII) can accommodate thousands of bats each, andhave been modified to fit three different bridgedesigns. Four of the five tested were fully occupied,one within the first month (Figure 20).

The Oregon Wedge (Figure 21) can house severalhundred bats and has been accepted for day roostingby 12 species, including a maternity colony of YumaMyotis (Myotis yumanensis) in Oregon (D. Clayton,pers. comm.). This design has been successful in bothbridges and culverts in Oregon, Arizona, and Texas.The Texas Department of Transportation developed aconcrete version that also attracted bats within a year(Figure 22).

Locations with evidence of attempted bat useare ideal for retrofitting projects. Roadways withstructures that pass through public lands, such asparks or national forests, are especially good candi-dates for bat habitat enhancement programs. Inmost cases, transportation department costs areminimal. In fact, local businesses are often willing todonate materials, assisting school children or private

Figure 19. Actual bridgeconstruction plan detailspecifying appropriate

width openings toaccommodate bats in aspecialized bridge type.

Courtesy of TomBarnard, LoBuono,

Armstrong & Associates.

1'-9"

5'-0"

Shldr

Bicycle/pedestrianrailing

park service rail7'-6" post spacing

3/4"-11/2" gap forbat habitat, typ

5'-0"

Shldr

2'-2"-2% -2%

1'-9"

PC/PSConcreteslabs

32'-0" clear roadway

L Bridge

11'-0"Lane

6" reinftopping

11'-0"Lane

C

(Spans 40'-70')3/16=1'-0"



agencies in constructing required structures. Newsmedia coverage and positive publicity of suchprojects has been extraordinary.

When 33,000 Mexican free-tailed bats became anuisance in the attic of the Canadian Middle Schoolin Canadian, Texas, teachers and students collaborat-ed on a project to provide alternate roosts for up to50,000 bats in a nearby highway bridge. They appliedfor, and received, an Environmental Challenge Pro-gram grant from the Texas General Land Office andH-E-B Grocery Company. Then they worked with BatConservation International and the Texas Departmentof Transportation to build and mount their roosts(Figure 23).

When old bridges must be replaced, some ofthose occupied by bats have been retained aswildlife sanctuaries. The Santa Barbara Public WorksDepartment and the California Department of Trans-portation are collaborating to preserve a colony of10,000 Mexican free-tailed bats and 200 pallid bats(Antrozous pallidus) by retaining a portion of an oldbridge that is surrounded by agricultural fields

Figure 20. Several designs of the Texas Bat-Abode, such asthis one modified for a steel I-beam bridge, have been usedto attract thousands of bats.

Figure 21. The Oregon wedge is inexpensive and easilyinstalled. This design has successfully attracted 12 speciesof bats.

Figure 22. The Texas Department of Transportationdeveloped a concrete bat house that provides homes forhundreds of bats.



(Storrer, 1994). It is calculated that these bats consumeroughly 10,000 pounds (4,540 kg) of insects eachsummer, many of which are pests.

In Oregon the Departments of Transportationand Fish and Wildlife have cooperated in retaininga bridge occupied by a colony of Yuma myotis thathad been slated for destruction (S. Cross, pers.comm.). Removal costs were avoided, while valuablewildlife habitat was protected.

Incorporating characteristics into new structuresspecifically for bats can be relatively inexpensive andeasy to do. The Texas Department of Transportationhas committed to construct a bat-friendly domedculvert (Appendix III). The cost to customize stan-dard culvert designs is minimal, and modificationscan even be implemented during construction(M. Bloschock, pers. comm.).

Bridge habitat enhancement techniques are alsobeing developed in other countries. In Australia, theroost portion of an old wooden bridge was retainedand incorporated into the underbelly of a newreplacement bridge (G. Hoye, pers. comm.). InEngland, special bat-friendly bricks and concretebat boxes have been provided to create roost spaces,and alterations to new bridge designs are beingused to incorporate bat habitat into bridges duringmitigation projects (Billington, 1997).

Figure 23. Canadian Middle School students worked with the Texas Department of Transportation toinstall Texas Bat-Abodes that can accommodate up to 50,000 bats.



Mitigation

TRANSPORTATION departments faced withbalancing human needs and sensitive wildlife

issues will find incorporation of bat roosts intohighway structures to be ideal for mitigation as wellas for proactive habitat enhancement. Roadwayconstruction negatively impacts bats both directlyand indirectly. Roads built along rivers or rock facescan permanently destroy roosts in cliffs or caveswithin or near the right of way. In addition, roadconstruction through riparian forests removes roost-bearing trees. Roads also increase human accessibilityto sensitive roosts in caves or mines, forcing bats toabandon these roosts when they are disturbed. It isessential to minimize environmental damage, espe-cially when state or federally listed endangeredspecies are present.

Unlike many other mitigation efforts, bat roostenhancement projects for roadways can be conductedonsite. As described in the previous section, there are

many options for helping bats in new or existingstructures. For example, while planning a highwaythrough the Tonto National Forest, the ArizonaDepartment of Transportation and the U.S. ForestService are collaborating on a project to incorporatebat habitat into a new highway bridge. The highwaydepartment is including mounting brackets in thebridge design plans, and the Forest Service isconstructing artificial roosts that the highwaydepartment will install (R. Orr, pers. comm.).

Another means of providing alternative roostsis by retrofitting nearby highway structures withhabitat or using free-standing bat house designs.There are commercially produced bat housesavailable that can accommodate up to tens of thou-sands of bats (see Bat Conservation International’sWeb site: www.batcon.org). These are ideal for usein off-site mitigation projects.



Commonly Asked QuestionsThroughout this project, careful consideration hasbeen given to questions of how bats in bridges orculverts affect people, the structures, and theenvironment. The following questions addressthose concerns.

What are the benefits of including bat habitat inhighway structures?The 1.5 million Mexican free-tailed bats from theCongress Avenue bridge in Austin, Texas consumeapproximately 10 to 15 tons of insects nightly, andthese include large quantities of the most costlyagricultural pests in the state (McCracken andWestbrook, in man.). The impact of even smallcolonies of bats in bridges can be considerable.Just 150 big brown bats (a common nationwidebridge-dweller) can consume enough adult cucumberbeetles in one summer to prevent egg-laying thatcould produce 33 million of their costly root-wormlarvae (Whitaker, 1995). Also, some insect peststend to avoid areas where bat echolocation callsare heard (Belton and Kempster, 1962; Agee, 1964).Press coverage of projects to incorporate bat habitatinto highway structures has been excellent andextremely positive.

Do bats affect structural integrity?During the nationwide surveys, no structuraldamage attributable to bats was observed, norwere any reports of such damage received. MarkBloschock, a Texas Department of Transportationbridge design engineer, inspected the CongressAvenue bridge and the University of Texas footballstadium and found no damage of consequencewithin the normal life span of concrete structures.The bridge has been occupied for more than 15 yearsby approximately 1.5 million bats, the stadium 63years by tens of thousands.

Organic materials that retain moisture, such as batdroppings, could facilitate oxidation on unprotectedmetal parts. Thus, bat roosts above exposed metalcomponents should be discouraged.

Do bat colonies in highway structures negativelyimpact the environment?During our nationwide surveys, no negative impactson natural or human environments were observed,nor were any reported. Even exceptionally large batcolonies numbering in the hundreds of thousands

have not been associated with environmentaldegradation. Two water quality studies were con-ducted on Town Lake beneath the Congress Avenuebridge bat roost by the City of Austin and the LowerColorado River Authority respectively. These studiesfound a negligible impact caused by the bat colony(Lyday, 1994; Guajardo, 1995). Large guano depositscan produce odors in the immediate vicinity that areunpleasant to some people, though there are fewcomplaints in Austin, despite having 1.5 million bats.

Will bat colonies interfere with maintenanceschedules?Bats roosting in highway structures are habituatedto vibrations and sounds associated with normaltraffic and will be minimally disturbed if mainte-nance operations create these conditions. Structuralmaintenance only affects bat colonies if the roost is

Figure 24. Keith Hutson, Alabama Game and FishDepartment, and Johnny Sims, Natchez Trace Parkway,inspect big brown bats raising young in an open crevice.Most bats leave in winter, but when summer work mustproceed, workers can protect bats in crevices with tarps.



suddenly exposed or if foreign materials (water, tar,gravel, etc.) are introduced. During our field surveys,we observed crews working on and around occupiedstructures without apparent effects on bats.

How can transportation departments minimizedisturbance to bat colonies in highway structures?Bats that occupy bridge crevices often ignore workersin the general area. Where work must be performedabove crevices that are open at the top (Figure 24),disturbance can be minimized by covering themwith tarps. Bats such as big-eared species, that roostin larger open areas between beams, are highly sus-ceptible to disturbance, but they typically do notoccupy bridges year round. Transportation depart-ments can avoid accidentally providing roosts wherebats are unwanted by minimizing the inclusion ofpreferred characteristics. (See page 11, CharacteristicsBats Prefer.)

Timing Maintenance ActivitiesIn most states, bats leave their summer bridge rooststo overwinter in more protected locations. Mainte-nance conducted between November 1 and February1 will minimize disturbance. In the southernmostregions, where freezing temperatures rarely occur forextended periods some bats may remain year round.Still, proceeding with winter maintenance activitieswill affect fewer bats and avoid the disturbance offlightless young that would occur in summer. Whenquestions arise, we recommend consultation withexperienced bat biologists.

ExclusionExcluding bats from a roost is a process that allowsthem to exit unharmed, but not re-enter. This reducesthe potential for humans to come in contact with bats.If maintenance work has to be done while bats are ina roost, exclusion may be necessary. To conduct anexclusion, primary exit points are identified andmarked. All other escape routes greater than 0.25 inch(0.6 centimeter) are sealed. Access to unused portionsof long crevices can be minimized by filling themwith suitable material, such as wood, backer rod,expanding foam, or caulk. Care should be taken toavoid sealing bats into the roost. A one-way valve isplaced over the primary exit points to prevent re-entry. Simple one way valves have been constructedusing wire mesh cones, PVC, and strips of clear plas-tic sheeting attached over exit points (Figure 25). Oncethe bats have been excluded, roost spaces can be per-manently filled with a suitable substance. Bats do notchew or remove materials. Bats displaced duringexclusions may try to return to the roost for a shorttime following the procedure.

The Florida Department of Transportation usedall aspects of this process during reconstruction ofa bat-occupied bridge. In order to minimize distur-bance to the bats, the project was initiated during thewinter months when the fewest bats were present.Properly sized wood strips were used to fill unusedportions of the roost crevice, and one-way valves con-structed of wire mesh were installed over theexit points. In this case, bats did not move into bathouses mounted on nearby poles within the project

Figure 25.

Wood blocks,expanding foam,or backer rod are

useful to sealcrevices.

One-wayvalves made

of wiremesh conesor PVC pipeare effective

over exitholes.

One-wayvalves made

of clearplastic

sheeting areeffective for

longcrevices.

Road Deck



period, but the department hopes that the bats willreturn to roosts being built into the new bridge.

Educating Maintenance WorkersBefore working near known roosts, maintenancecrews should be taught not to handle bats and howto avoid disturbing them (Figure 26). Educationalmaterials can be obtained by contacting stategame and fish departments or Bat ConservationInternational, Inc.

Do bat colonies in a bridge or culvert pose humanhealth or safety risks?Most small bridge bat colonies pose no threatto humans and probably will remain unnoticedthroughout the life of the structure. However,spectacular emergences of large bat colonies fromhighway structures can attract public attention,as has been demonstrated at the Congress Avenuebridge. Tens of thousands of visitors have cometo view this spectacle each summer for more thana decade. Measures to minimize human contactas well as signs warning about handling bats maybe needed at heavily visited locations. Even though

the Congress Avenue bridge is located in the midst ofa large metropolitan area, no one has contracted anydisease from the 1.5 million bats in the 16 years sincethey arrived. A fence prevents access to areas whereyoung or sick bats sometimes fall, and signs warnvisitors not to handle bats.

Only two diseases, rabies and histoplasmosis,have been transmitted from bats to humans, andexposure risks are easy to avoid. Rabies can betransmitted only from the bite of a rabid animalor from contact between an infected animal’s nervetissue and an open wound. The virus is not foundin urine or feces. The occasional bat that doescontract rabies is almost never aggressive andbecomes a problem only if handled. Any animalbite should be professionally evaluated as a potentialrabies exposure. A safe, effective, and painless vaccineis now available, for either pre- or post-exposureprotection.

Histoplasma capsulatum is a fungus that livesin soil enriched by animal droppings and can causea respiratory illness called histoplasmosis, which ismost often contracted from birds. Humans risk infec-tion only when they inhale spore-laden dust.

Figure 26. The Georgia Department of Transportation is learning about bats to help avoid problemsduring maintenance activities.



Bridge workers should minimize dust inhalationwhere there are either bird or bat droppings. Arespirator capable of filtering 2 to 3 micron-sizedparticles should be worn in work areas where inhala-tion of dust from animal droppings cannot beavoided (Kunz, 1998).

How do endangered or threatened bat species affecttransportation departments?Transportation departments can often mitigate alter-ation of sensitive roost habitats by providing spacefor bats in highway structures. There are currentlysix federally endangered bat species on the U.S.mainland. The gray myotis (Myotis grisescens) hassuccessfully used both bridges and culverts asmaternity roosts. The Indiana myotis (Myotis sodalis)has been documented to use bridges as day roosts,but bridge suitability for this species remains poorlyinvestigated. Although the two endangered big-earedbat subspecies (Corynorhinus townsendii virginianus andC. t. ingens) have not been documented in highwaystructures, western big-eared bats regularly usebridges as day roosts. Endangered lesser (Leptonycteriscurasoae) and greater long-nosed bats (Leptonycterisnivalis) found in the extreme southwestern U.S., havenot been documented using highway structures.

In contrast to other endangered plant and animalspecies, bats have a mobility and behavioral adapt-ability that allows greater bridge maintenance andreplacement flexibility. Bridges or culverts occupiedby endangered bat species often can be worked onwithout disturbing the bats by simply choosing a timewhen bats are not present. Varied mitigative measuresare also available (see Mitigation).

How are transportation departments dealing withbats in highway structures?The Federal Highway Administration was the leadagency initiating the national study of bat use ofbridges followed by contributions from Texas, Florida,Georgia, Tennessee, Oklahoma, Wyoming, Utah, andNew Mexico transportation departments. Individually,growing numbers of transportation departments areintegrating bat management techniques into mainte-nance schedules. California evaluates every projectfor impacts to bats. Significant local wildlife resourcesand species of concern listed by the state or federalgovernments are given special consideration(G. Erickson, pers. comm.). The Arizona Department

of Transportation also includes bats in its environ-mental impact statements with an emphasis onspecies of concern (T. Snow, pers. comm.).

The Texas Department of Transportation hasconducted a statewide study of bat use in highwaystructures and is using the information to activelypreserve and promote bat roosts where appropriate.Thousands of bats have new homes throughout thestate in both bridges and culverts retrofitted with batroosts. In south Texas, methods of trimming palmtrees within the right of way have been altered toretain dead fronds where bats are roosting.

How important are highway structures to bats?In many cases, bridges and culverts now serve ashavens of last resort for bats that have lost theirnatural roosts in caves and old-growth forests.However, surrounding habitat often remains suitable,if only bats can find safe places to rear their young.Typically, where traditional roosts have beenprotected, or new ones have been provided,even endangered species are recovering.

Though less than one percent of Americanbridges are currently suitable for use by bats, thesebridges already shelter millions of bats of at least24 species, including some of our continent’s mostregionally important populations. The fact that batswere often found attempting to rear young in sitesunprotected from rain, or where many were killed bypassing cars as they emerged, demonstrates that roostshortages are common.

Roost loss and disturbance are the most impor-tant known causes of bat decline. Yet, as we havedocumented, bridges and culverts can provideessential substitutes. Transportation departmentsare ideally positioned to help reestablish one ofAmerica’s most valuable wildlife resources at littleor no cost to taxpayers, through highly popularproactive measures.

Bats are often forced into dangerous conditionswhen safe roosts are in short supply. In one instance,bats were found emerging from a bridge locatedover a busy highway where they were frequentlyhit by cars. In another instance, several hundredMexican free-tailed bats died apparently fromhypothermia during an early winter cold front,when rain leaked into an unsealed crevice soakingthem. These incidents emphasize the importance ofproviding adequate conditions when planning habitatenhancement for bats.



Discussion

IT IS ESTIMATED that within the southern U.S.,3,600 highway structures are being used by

approximately 33 million bats. The fact that 43percent of bridges suitable for night roosting areused, indicates that in many areas bat habitatenhancement projects would be successful andcould help stabilize bat populations by providingroosts needed for rearing young.

Other countries are also beginning to recognizethe value of providing roosts in bridges and areinitiating their own projects. Information from theBats in American Bridges project has already been

requested from 17 countries, suggesting that habitatenhancements in highway structures may becomea powerful conservation tool worldwide.

As illustrated at the Congress Avenue bridge,the public has firmly demonstrated its support forbats in highway structures. Furthermore, researchdocumenting the impact of bats in reducing croppests is rapidly increasing support in the agriculturalcommunity. People support what they value, and therelationship between bats and highway structures isclearly valuable to both humans and bats.

Common Name D o c u m e n t e d P o t e n t i a l Roost type P re f e re n c e s U . S . S t a t u sb r i d g e / c u l v e rt u s e c revices or d i s t r i b u t i o n

u s e open beams

Peters’ ghost-faced bat no yes SWMormoops megalophylla

Mexican long-tongued bat yes open sheltered SW Species ofChoeronycteris mexicana spaces Concern

Lesser long-nosed bat no yes SW EndangeredLeptonycteris curasoae

Long-nosed bat no yes SW EndangeredLeptonycteris nivalis

California leaf-nosed bat yes open sheltered SWMacrotus californicus spaces

Pallid bat yes crevice 1 to 1.5 SWAntrozous pallidus inches

Rafinesque’s big-eared bat yes open sheltered E Species ofCorynorhinus rafinesquii spaces Concern

Townsend’s big-eared bat yes open sheltered NW, SW Species ofCorynorhinus townsendii spaces Concern

Virginia big-eared bat no yes open sheltered E EndangeredCorynorhinus townsendii virginianus spaces (subspecies)

Ozark big-eared bat no yes open sheltered SC EndangeredCorynorhinus townsendii ingens spaces (subspecies)

Big brown bat yes crevice 0.75 to 1.5 NationwideEptesicus fuscus inches

Spotted bat no yes NW, SW Species ofEuderma maculatum Concern

Allen’s lappet-browed bat no yes SW Species ofIdionycteris phyllotis Concern

Silver-haired bat yes crevice 0.75 to 1.25 NE, NW,Lasionycteris noctivagans inches C, S, W

Southwestern myotis no yes SWMyotis auriculus

Southeastern myotis yes open/ 0.5 to 1.0 SE Species ofMyotis austroriparius crevice inch Concern

California myotis no yes NW, SWMyotis californicus

Western small-footed myotis yes crevice 0.5 to 1.0 NW, SW Species ofMyotis ciliolabrum inch Concern

Long-eared myotis yes crevice 0.5 to 1.25 NW, SW Species ofMyotis evotis inches Concern



Appendix I:

Bats that Use Bridges and Culverts

Gray myotis yes crevice 0.5 to 1.0 NE, SE EndangeredMyotis grisescens inch

Keen’s myotis no yes crevice 0.5 to 1.0 NWMyotis keenii inch

Small-footed myotis yes crevice 0.5 to 1.0 NE Species ofMyotis leibii inch Concern

Little brown myotis yes crevice 0.5 to 1.0 NE, SE,Myotis lucifugus inch NW, SW

Eastern long-eared myotis yes crevice 0.5 to 1.0 NEMyotis septentrionalis inch

Indiana myotis yes crevice 0.5 to 1.0 NE EndangeredMyotis sodalis inch

Fringed myotis yes crevice 0.5 to 1.0 NW, SW Species ofMyotis thysanodes inch Concern

Cave myotis yes crevice/open 0.5 to 1.0 SW Species ofMyotis velifer inch Concern

Long-legged myotis yes crevice 0.5 to 1.0 NW, SW Species ofMyotis volans inch Concern

Yuma myotis yes crevice 0.5 to 1.0 NW, SW Species ofMyotis yumanensis inch Concern

Evening bat yes crevice 0.5 to 1.0 NE, SENycticeius humeralis inch

Western Pipistrelle yes crevice/open 0.5 to 1.0 NW, SWPipistrellus hesperus inch

Eastern Pipistrelle yes open NE, SEPipistrellus subflavus

Florida mastiff bat no yes Florida Species ofEumops glaucinus Concern

Western Mastiff bat no yes SW Species ofEumops perotis Concern

Underwood’s mastiff bat no yes SW Species ofEumops underwoodi Concern

Pallas’ mastiff bat no yes FloridaMolossus molossus

Pocketed free-tailed bat no yes SWNyctinomops femorosaccus

Big free-tailed bat yes crevice 1 to 1.5 SW Species ofNyctinomops macrotis inches Concern

Mexican free-tailed bat yes crevice/open 0.5 to 1.25 SE, SWTadarida brasiliensis inches



Common Name D o c u m e n t e d P o t e n t i a l Roost type P re f e re n c e s U . S . S t a t u sb r i d g e / c u l v e rt u s e c revices or d i s t r i b u t i o n

u s e open beams



Appendix II:

Survey Forms

THE FOLLOWING FORMS may prove useful in evaluating highway structures for actual or potential bat use.Some field experience may be necessary prior to initiating the surveys to develop familiarity with potential

roost locations within structural designs. June is the best time of the year to conduct surveys, since nurserycolonies are most detectable when rearing young.

D a t e S t a t e C o u n t y H i g h w a y H i g h w a y H i g h w a y H i g h w a y L a t i t u d e L o n g i t u d e A l t i t u d e E c o l o g i c a lt y p e : t y p e : t y p e : t y p e : re g i o n

I n t e r s t a t e U.S. Hwy. S t a t e C o u n t y

Location

123456789

101112131415161718192021222324252627282930



123456789

101112131415161718192021222324252627282930

B a t s S p e c i e s # of # of D a y N u r s e ry N i g h t # of R o o s t Roost type: Roost type: Roost type:p re s e n t b a t s s p e c i e s ro o s t ro o s t ro o s t ro o s t s t y p e : p l u g g e d s w a l l o w i m p e rf e c t i o ny e s / n o y e s / n o y e s / n o i n t e n s i t y c re v i c e d r a i n n e s t s or other

Bridge Night Roost Index:

0 No sign of droppings or urine stains.

1 Small amount of such signs in only one location.

2 Small urine stains and scattered droppings in several locations.

3 Moderate dropping accumulations. Urine stains obvious within the bridge.

4 Large dropping accumulations. Fresh urine stains obvious and widespread.

5 Dropping accumulations several inches thick in several locations. Roosting evidentthroughout structure. Fresh urine stains in all optimal locations.

Bats/Roost Types



Parallel P re - Cast in S t e e l Flat slab O t h e r : C o n c rete box culvert : C o n c re t e M e t a l Ideal* cre v i c e sb o x s t re s s e d p l a c e I - b e a m s p e c i f y # of barre l s round culvert s : c u l v e rt : p re s e n t

b e a m g i rd e r and height d i a m e t e r d i a m e t e r y e s / n o

Structure Design

123456789

101112131415161718192021222324252627282930

* Evidence ofstorm-waterstainingalong thelength of thecrevice indi-cates that itis not sealedand shouldnot be con-sidered asan idealroost.

Figure 28. Ideal Highway Structure Roost CharacteristicsFigure 27. Cross-sections of Common Bridge Designs

Parallel boxbeam

Prestressedconcretegirder

2 commoncast-in-place

Steel I-beam

2 commonflat slabdesigns,occasionallywith hollowchambers

Water-tight seal

Full sun for amajority ofthe day

Crevices0.75 to 1 inch wide

(1.9 to 2.5 centimeters)10 feet

(3 meters) ormore above the

ground

Not situated overbusy roadways

Crevices12 inches(30 cm)

or greaterin depth

Concrete asprimary

constructionmaterial

Located inthe southern

half of theU.S.



Roost Substrate and Dimensions

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R o o s t H e i g h t H e i g h t C re v i c e C re v i c e C re v i c e C re v i c e C re v i c e C re v i c e C re v i c e R o o s t R o o s t R o o s tH e i g h t M a x . M i n . R o o s t Wi d t h Wi d t h D e p t h D e p t h D e p t h L e n g t h S u rf a c e S u rf a c e S u rf a c e

Av. Width Av. M a x . M i n . Av. M a x . M i n . C o n c re t e M e t a l O t h e r



Surrounding Habitat

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R e s i d e n t i a l A g r i c u l t u re C o m m e rc i a l Wo o d l a n d G r a s s l a n d R a n c h i n g R i p a r i a n M i x e d



Conditions Beneath the Roost

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B a re O p e n C l o s e d F l o w i n g S t a n d i n g 4-lane + 2 - l a n e D i rt ro a d R a i l ro a d C o n c re t eg ro u n d v e g e t a t i o n * v e g e t a t i o n† w a t e r w a t e r h i g h w a y h i g h w a y

* Vegetation not blocking flight path within 10' of bridge underside or more than one entrance of a culvert.† Vegetation interfering with bat access to potential roosts, either blocking bridge underside or both ends of

culverts.



Appendix III:

Retrofitting for Bats

BAT-FRIENDLY HABITAT can be provided ineither new or existing bridges or culverts, at little

or no extra cost to taxpayers. During constructionplanning, there are no costs for an engineer to specifythe appropriate crevice widths of 3/4 to 1 inch (1.9 to2.5 centimeters) for expansion joints or other crevices.Existing structures can be retrofitted with bat-friendlyhabitats using the designs described in the followingsections. All retrofitting activities must be coordinatedwith appropriate transportation departments. Signs of batuse in nearby bridges and culverts increase thechances of success for habitat enhancement projects.

The Texas Bat-Abode, Big-eared Bat-Abode,and the Oregon Bridge Wedge bat roosts aredesigned for day-roosting bats in bridges andculverts. In the protected environment of a bridgeor culvert, a properly constructed and installed bathabitat made of quality materials should last aslong as the highway structure.

Texas Bat-AbodeThe Texas Bat-Abode is designed to retrofit bridgeswith bat habitat for crevice-dwelling species. It hasan external panel on either side and 1 by 2 inch (2.5to 5.1 centimeter) wooden spacers sandwichedbetween 0.5 to 0.75 inch (1.2 to 1.9 centimeters) ply-wood partitions (Figure 29). Recycled highway signsare ideal construction materials. Note that only theexternal panels need to be cut to fit the bridges’ inter-beam spaces. The internal partitions should providecrevices 0.75 inch (1.9 centimeters) wide and at least12 inches (31 centimeters) deep.

Smooth roost surfaces need to be textured toprovide footholds for bats on at least one side ofeach plywood partition (preferably both), creatingirregularities at least every 1/8 inch (0.3 centimeter).Many methods have been tested to create footholds,such as:• using rough-sided paneling• coating the panel with a thick layer of exterior

polyurethane or epoxy paint sprinkled withrough grit

• attaching plastic mesh with silicone caulk orrust-resistant staples

• mechanically scarifying the wood with a sharpobject such as a utility knife

• lightly grooving the wood with a saw (do notpenetrate to the first plywood glue layer)

• lightly sandblasting the wood with rough-grit

The use of rough-sided paneling or polyurethane-sprinkled with grit have provided the longest lastingresults. Rust resistant wood screws should be usedto assemble the spacers and partitions.

The Texas Bat-Abode should be installed inbridges that are at least 10 feet above ground, freeof vegetation, and not susceptible to flooding or easyvandalism. Measurements of the exact location wherethe Bat-Abode is to be placed will ensure a proper fit.The number of partitions is arbitrary and limited onlyby availability of materials and support for theweight of the Abodes. Because of the weight, it maybe easiest to assemble the cut pieces in the bridge.In wooden bridges, the unit should be anchored tothe structure with heavy-duty rust-resistant lag-bolts.

Big-eared Bat-AbodeBig-eared bats are frequent bridge users in both theeastern and western United States. They prefer openroost areas such as cave entry rooms, large hollowtrees, darkened undisturbed rooms in abandonedhouses, or between the darkened beams of quietbridges over streams. The Big-eared Bat-Abodecreates these conditions.

The Big-eared Bat-Abode has two external panelswith 1 by 2 inch spacers that are used as braces to

Figure 29. Texas Bat-Abode for crevice-dwelling species.

bridge deck

bridgebeam

x-section

bridgebeam

x-section

at least12" deep(31 cm)

1x2" spacer

fullbeam

height



hold the panels together with a plastic mesh liningto provide footholds for bats. The netting shouldbe attached using rust-resistant staples (Figure 30).The other methods of creating footholds men-tioned above would also be effective.

It may be easier to partially assemble the struc-ture on the ground leaving one end panel off untilit is placed in its chosen location. Units installed inwooden bridges can be anchored using heavy-dutyrust-resistance lag bolts. Because big-eared bats arevery sensitive to disturbance, units should beplaced in areas of low activity and painted a colorthat does not attract attention.

Big-eared bats are often found in low bridgesdarkened by thick vegetation growing along thesides. The Big-eared Bat-Abode should be placedat least six to 10 feet (two to three meters) abovethe ground in a secluded portion of the bridge.However, access to the fly-way entrance shouldnot be blocked. Other bat species are also likely touse this structure.

The Oregon WedgeThe Oregon Wedge (Figure 31) is an inexpensivemethod of retrofitting bridges or culverts withday-roost habitat for bats. The Wedge is made froman 0.5 to 0.75 inch (1.2 to 2 centimeters) exterior gradeplywood panel that is at least 18 inches high and 24inches wide (46 by 61 centimeters) with three 1 by 2inch (2.5 by 5 centimeters) wood strips attached alongthe top and sides, leaving an opening along the bot-tom.If larger panel sizes are used, vertical wooden piecesshould be placed every 24 inches (61 centimeters) tosupport the plywood and prevent warping. Thepieces should not run from the top to the bottom sothat bats can move about within the panel.

The Wedge can be attached to a vertical concreteportion of a bridge or culvert using concrete anchor-bolts or a fast-drying environmentally safe epoxycement (such as 3M Scotch coat 3-12). The transporta -tion department should install the panels if anchor boltsare used. If the panel is to be attached to wood, thenuse appropriate rust resistant wood screws. Beforeapplying the epoxy, check the preferred installationsite to make sure the support strips fit flat against theconcrete surface.

Wedge placement is possible on any adequatelysized, flat concrete or wood surface. However, we

Figure 30. Big-eared Bat-Abode.

Figure 31. Oregon Bridge Wedge. Designs courtesy of DavidClayton and Dr. Steve Cross.

24"(61 cm)

18"(46 cm)

1 by 2 inch (2.5 by 5 centimeters)wooden strips provide ideal spacing

bridge deck

bridgebeam

x-section

bridgebeam

x-section

2'(61 cm)



recommend that the panels be placed near the sun-warmed road slab (preferably as high as possiblebetween heat-trapping bridge beams). They shouldbe at least 10 feet (3 meters) above ground, with aclear flyway (at least 10 feet), and be out of view orreach of vandals. The Wedge can also be installed inthe middle sections of culverts higher than 5 feet(1.5 centimeters). A Wedge should not be placed instructures that flood. As a precaution against flood-ing, a 1.5 inch (3.8 centimeters) gap can be left ateach corner where the support strips join to act asan escape route in the event of fast-rising water.

Bat-domed CulvertsThe Bat-domed culvert (Figure 32) is a modifiedconcrete box culvert designed to accommodate largecolonies of bats. The dome has several bat-friendlycharacteristics:• the height is increased• warm air is trapped• light intensity is reduced• air movement is reduced

Figure 32. Bat-domed culvert. Graphics courtesy of TexasDepartment of Transportation.

N o t e : The 3M epoxy can beobtained by calling 1-800-722-6721.BCI would appreciate photographsof the installation and especially ofbats using any bat-friendly modifi-cations. For more informationon adapting the designs to specificbridges, or to report occupiedunits, please contact: Brian Keeley,Bats and Bridges Project Coordina-tor, Bat Conservation International,Inc., P.O. Box 162603, Austin,Texas 78716. Phone (512) [email protected]

Bat-domed culverts should be at least 5 feet(1.5 meters) in height with an additional 1 to 2-foot(0.6 meter) raised portion centered in the culvert.The raised area can be any length from 2 to 50 feet,depending on the colony size preferred. The wallsand ceilings of the raised area should be roughenedto provide footholds for bats. The following methodwas used to produce suitable wall and ceilingtextures. Using a crowbar, thin strips were removedfrom the surface of recycled plywood. The resultingroughened wood was then used as the form for pour-ing the concrete, which produced the desiredtextured surface within the domed area of theculvert. In addition, a method of attaching panelsor partitions, such as female threaded inserts, canbe incorporated into the raised walls and ceilingto create more surface area once the culvert iscompleted.

Bat-domed culverts should not be placed inareas susceptible to flooding. However, in the eventof rising water, the dome may serve as a temporaryair-trap. Almost any cave-dwelling species mayuse these, including several that are endangered.



Appendix IV:

Useful ReferencesAgee, H.R. 1964. Response of flying bollworm moths and other tympanate moths to pulsed ultrasound. Annals

of the Entomological Society of America 62:801-807.Adam, M.D. and J.P. Hayes. 1996. Use of bridges by bats as night roosts in the Oregon coast range. Suislaw

National Forest, U.S. Forest Service and Bureau of Land Management, Eugene district, Oregon, 46 pp.Bailey, R.G. 1995. Description of the ecoregions of the United States. Miscellaneous Publication. No. 1391.

United States Department of Agriculture: Forest Service, Washington, D.C., 108 pp.Barbour, R.W. and Davis, W.H. 1969. Bats of America. University of Kentucky Press, Lexington, Kentucky, 286pp.Belton, P., and R.H. Kempster. 1962. A field test on the use of sound to repel the European corn borer.

Entomologia Experimentalis at Applicate 5:281-288.Billington, G. and G. Norman. 1997. The conservation of bats in bridges project. A report on the survey and

conservation of bat roosts in Cumbria, England. Lake District National Park, Cumbria, England, 97 pp.Davis, R. and E.L. Cockrum. 1963. Bridges used as day-roosts by bats. Journal of Mammalogy 44:428-430.Guajardo, J. 1995. Letter to Bat Conservation International: Water quality lab results of Town Lake. Lower

Colorado River Authority, Austin, Texas, 5 pp.Hirschfeld, J.R., Z.C. Nelson, and W.G. Bradley. 1977. Night roosting behavior in four species of desert bats.

The Southwestern Naturalist 22:427-433.Keeley, B.W. 1994. Research begins on bat friendly bridges. Bats 12(4):18-19.____. 1997. Bats in bridges. Bats 15(3):8-10.____. 1998. Bat use of bridges. Bureau of Land Management, Coos Bay District, and Oregon Department of

Fish and Wildlife, Coos Bay, Oregon, 15 pp.Keeley, B.W., and M.D. Tuttle. 1996. Texas bats and bridges project. Texas Department of Transportation,

Austin, Texas, 16 pp.Kunz, T.H. 1982. Roosting ecology of bats. Pp.1-55 in Ecology of Bats (T.H. Kunz, ed.). Plenum Publishing,

New York, 425 pp.Lyday, M. 1994. Interoffice memo; Subject: Bat scat impacts to Town Lake around Congress Avenue Street

bridge. City of Austin Resource Evaluation Section, Austin, Texas, 2 pp.McAney, K. 1992. Bats and bridges: A report on the importance of bridges to bats. National Parks and Wildlife

Service, Galway, Ireland, 11 pp.McCracken, G.F. 1986. Why are we losing our Mexican free-tailed bats? Bats 3(3):1-4.____. 1996. Bats aloft: A study of high-altitude feeding. Bats 14(3):7-10.

Mishler, C. 1991. Bats and the “A” Street trestle. Outdoor California 52 (3):6-8.Mitchell-Jones, A. 1989. Bridge surveys. Batchat 12:5.Mumford, R.E. and J.B. Cope. 1958. Summer records of Myotis sodalis in Indiana. Journal of Mammalogy

39:586-87.Norman, G.M. 1995. Bats and bridges in Cumbria. The Carlisle Naturalist 3:36-37.Perlmeter, S.I. 1995. Bats and Bridges: Patterns of night roost use by bats in the Willamette National Forest. Bat

Research News 36(2 & 3):30-31.Pierson, E.D., W.E Rainey, and R.M. Miller. 1996. Night roost sampling: a window on the forest bat community

in northern California. Pp.151-163, in Bats and Forests: Proceedings of the Victoria Symposium. (R.M.R.Barclay and R. M. Brigham, eds.). Ministry of Forests, British Columbia, 292 pp.

Roberst, D. 1989. Bats under bridges in North Yorkshire. Bat News 16:6-7Smiddy, P. 1991. Bats and bridges. Irish Naturalist Journal 23:425-426.Storrer, John. 1994. Characterization of the Garey bridge bat roost Santa Barbara County, California. Appendix

V, in Natural Environment Study Report, Garey bridge replacement project, Santa Barbara County,California. Santa Barbara Department of Public Works, Santa Barbara, California, 24 pp. + appendices.



Schwarting, Von H. 1995. Fledermäuse in nordamerikanischen Straßenbrücken sowie Einblicke in die Fledermausfauna von Punta Gorda/Südwest-Florida. Nyctalus (N.F.) 5:421-440.

Vaughan, T.A. and T.J. O’Shea. 1976. Roosting ecology of the pallid bat, Antrozous pallidus. Journal ofMammalogy 57:19-42.

Winslow, A. 1995. The provision of bat roosting sites in tunnels and bridges. Bat News 38:3.Wilkins, K.T. 1989. Tadarida brasiliensis. Mammalian Species 331:1-10.Whitaker, J.O., Jr. 1995. Food of the big brown bat Eptesicus fuscus from maternity colonies in Indiana and

Illinois. American Midland Naturalist 134:346-360.Whitaker, J.O., Jr., C. Maser, and L.E. Keller. 1977. Food habits of bats in western Oregon. Northwest Science

51: 46-55.



Appendix V:

State Survey ResultsStates field Total number Total Totalsurveyed of highway structures occupied night roosts

surveyed day roosts encountered

Alabama 13 5 1

Arizona 101 9 57

Arkansas 49 6 8

California 48 11 19

Colorado 50 0 31

Florida 142 29 22

Georgia 107 9 15

Idaho 30 2 9

Kansas 18 3 8

Kentucky 29 10 20

Louisiana 41 6 16

Mississippi 58 1 11

Missouri 37 2 17

Nevada 18 4 5

New Mexico 39 7 20

North Carolina 35 6 6

Oklahoma 66 0 13

Oregon 98 15 73

South Carolina 44 0 16

Tennessee 66 7 25

Texas 1060 62 156

Utah 58 4 40

Virginia 74 10 27

Washington 19 0 12

Wyoming 121 3 87

Total 2421 211 714



Appendix VI:

Ecoregions of the United States

Bat Conservation International, Inc.

P.O. Box 162603Austin, Texas 78716

Bats Bridges 2

Documents

bat conservation international

bat habitat

bat roosts

highway structures

conservation information

public safety

subsequent retrofitting

duringinitial construction