Emily Dickinson

Emily Dickinson

Why do birds migrate?

•Migration is a strategy to take advantage of seasonally abundant food supplies

•Weather and photo-period are triggers

SNOW BUNTING

DECJAN

FEB

MAR

APR

MAY

NOV

OCT

SEP

AUG

Complete Migration

•All individuals leave the breeding areaafter raising their families

•Most complete migrants breed in temperate northern climates

•Many complete migrants travel incredible distances between breeding andnon-breeding habitats

Cerulean warbler

Wilson’s plover

Stilt sandpiper

MIGRATION STRATEGIES

Partial Migration

•Seasonal movement by some, but not all, individuals away from breeding area

•Some overlap of breeding & non-breeding ranges

Red-tailed hawk

Herring gull Bewick’s wren

MIGRATION STRATEGIES

Irruptive Migration

•Not seasonally or geographically predictable

•Distances and number of migrants areunpredictable

•Food specialists leave the boreal forests for more southerly ranges when food isn’t available

Pine siskin Red-breasted nuthatch

MIGRATION STRATEGIES

MOON WATCHINGRequires a full moon, a comfortablechair and binoculars

How dowe study migration?

BANDINGUS Fish and Wildlife Service BirdBanding LaboratoryRADAR (NOAA)Several years ago, researchers at Cape May counted 14 million birds in one nightRADIO TELEMETRYCarl Safina followed a single radio-tagged albatross throughout its range

27 192 358 6792,100

4,615

8,094

15,000

32,322

42,868

52,471

0

10000

20000

30000

40000

50000

60000

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

How dowe study migration?

Number of participants 1900-2000

CHRISTMAS BIRD COUNT

52,471

Anatomy of Flight

• A fused sternum with a “keel” provides the attachment points for the flight muscles.

THE BASICS OF FLIGHT

A flap is a flap

• Flight feathers are asymmetrical to allow them to change shape during flight• A wing must allow air to pass through it on the up-stroke

THE BASICS OF FLIGHT

Higher Pressure

LIFT

Eddy = DRAG

Airflow

Lift & dealing with drag

•Lower air pressure above and higher pressure beneath wing creates lift

•During flight, air currents coming off the trailing edge of the wing create eddies which cause drag

Wing cross-Wing cross-sectionsection

DEALING WITH DRAG

Wing slotting

One way to reduce the drag causedby these eddies is to break them up into smaller eddies.

DEALING WITH DRAG

Primary feathers create smaller eddies = less drag

Ground effects

Flying very close (within a wing length) to a body of water allows the rippled surface to absorb the air currents coming off the trailing edge of the wing, reducing drag.

DEALING WITH DRAG

Powered•Characterized by continuous flapping•Results in a level course through the air

•Examples: sandpipers, ducks, geese, rails and hummingbirds

THE BASICS OF FLIGHT

Bounding

•Birds flap in short bursts to gain altitude, and then descend with wings folded against the body

•Bounding flight results in constant climbingand descending

•Examples: warblers, vireos, small woodpeckers, orioles, robins and tanagers

THE BASICS OF FLIGHT

Partially powered gliding

•Partially powered gliding birds alternate between flapping their wings and holding them extended

•Small hawks use flapping to fly between thermals

•Examples: cranes, swallows, swifts, pelicans, and shearwaters

THE BASICS OF FLIGHT

Gliding birds keep their wings extendedand ride rising currents of warm air tostay aloft. They flap only to regain altitude.

Ratio measures horizontal distance to vertical drop

SAILPLANE SAILPLANE 60:160:1ALBATROSS ALBATROSS 20:120:1HAWK 10 to HAWK 10 to 13:113:1

MONARCH BUTTERFLY 3:1MONARCH BUTTERFLY 3:1

Gliding

THE BASICS OF FLIGHT

Route finding

•Birds navigate by: visual landmarks, the sun, moon, stars, and routes learned from other birds

•Birds also have an internal compass which issensitive to the Earth’s magnetic field

•Migrating birds will maintain a true compass heading

MOON

LANDSCAPEFEATURES

SMELLS

WINDS

STARS

MAGNETICNORTH

SUNRISE

WEATHERUV LIGHT

SOUND

Birds usually select the mostefficient flight speeds.How fast?

SEMI-PALMATED PLOVERSEMI-PALMATED PLOVER 22-3222-32BALTIMORE ORIOLEBALTIMORE ORIOLE 22-3022-30BLACKPOLLBLACKPOLLWARBLERWARBLER 15-2315-23

28-4028-40GOLDEN PLOVERGOLDEN PLOVER

30-4430-44BROAD-WINGED HAWKBROAD-WINGED HAWK

32-4632-46OSPREYOSPREY

28-5028-50COMMON LOONCOMMON LOON

0 50 mph10 20 30 40

How long?

•Migrants using powered flight regularly cross stretches of open water like the Gulf of Mexico (500 miles)

•Others flap continuously for as long as 70 hours

•Examples: Ruby-throated hummingbird, Wood thrush and Blackpoll warbler

How far?NORTH AMERICAN MIGRATION FLYWAYS

Atlantic FlywaysMississippi Flyways

Central FlywaysPacific Flyways

How far?

Red KnotFlies from Argentina to Brazil, to the Delaware Bay,to the Arctic.

7,000 miles one way

How far?

Blackpoll warblerWinter in South America, and flyto the Caribbean,and then to northern breeding grounds.

2,500 to5,000 miles

one way

How far?

HummingbirdsWinter in Central America and fly as far north as Canada.

Some fly across the Gulf of

Mexico. (500 miles nonstop)

How far?

Blue-winged TealWinters in South America, and breeds in northern plains

7,000 milesone way

Diurnal migrants

•Most birds which rely on gliding or flap& glide flight are diurnal migrants

•The chief benefit of daytime flight is thermals – rising currents of warm air that provide lift

DAY OR NIGHT?

Nocturnal migrants

•Many birds which use powered flightare nocturnal migrants. Why…?

•Night flying helps birds avoid predators

•Other benefits include calmer, more stable air

DAY OR NIGHT?

Fat is the currency of migration

•Fat is the most important fuel for migration

•Many species double their weight with fat fuel for migration

Shorebird Songbird HawkTypical Body Fat: PRE-MIGRATION

66% 70% 15%

Fat is the currency of migration

PRE-MIGRATION(Brazil)

POST-MIGRATION(Delaware Bay)

RED KNOT BODY FAT3%

66%

Re-fueling stations are critical

When a Red Knot arrives at the Delaware Bay,in two weeks it must increase its body weightby 60% to complete the trip to its arctic feeding grounds. The additional weight must be in the form of fat…

A 175 lb man who wants to gain 105 lbs (60%) in two weeks would have to eat 46 Big Macs per day for 14 days…

…and turn itall into fat!

Birds rely on the samere-fueling stations

Stopover places provide abundant foodfor re-fueling needed by shorebirds to complete migration, as well as roosting places.

Copper RiverDelta, AK

San FranciscoCA

Gray’s Harbor,WA

CheyenneBottoms,

KS

Platte River,NE

James Bay,Canada

DelawareBay, MD

Bay ofFundy, ME

Threats to migration

•Loss of non-breeding ranges due to agricultureand seaside development

•The destruction of the tropical and boreal forests

•Habitat fragmentation

The biggest threat to migration is habitat loss

Everythingin the worldis connected to everything else

•Migratory routes are the invisible lines ofconnection which show us how people,places, and wildlife depend on each other

•Migration tells us about our physicaland spiritual health

Sources & further reading

OrnithologyFrank B. Gill, W.H. Freeman & Co, 1995 The Random House Atlas of Bird MigrationJonathan Elphick, ed., Random House, 1995The Flight of the Red KnotBrian Harrington, W.W. Norton & Company, 1996How Birds MigratePaul Kerlinger, Stackpole Books, 1995The Audubon Encyclopediaof North American BirdsJohn K. Terres, Alfred A. Knopf, 1982Living on the WindScott Weidensaul, North Point Press, 1999

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