Emily Dickinson
Why do
birds
migrate?
• Migration is a strategy to take advantage
of seasonally abundant food supplies
• Weather and photo-period are triggers
DEC
NOV
JAN
OCT
FEB
SEP
MAR
AUG
APR
MAY
SNOW BUNTING
MIGRATION STRATEGIES
Complete
Migration
Cerulean warbler
Wilson’s plover
Stilt sandpiper
• All individuals leave the breeding area
after raising their families
• Most complete migrants breed in
temperate northern climates
• Many complete migrants travel incredible
distances between breeding and
non-breeding habitats
MIGRATION STRATEGIES
Partial
Migration
Red-tailed hawk
Herring gull
Bewick’s wren
• Seasonal movement by some, but not all,
individuals away from breeding area
• Some overlap of breeding & non-breeding
ranges
MIGRATION STRATEGIES
Irruptive
Migration
Pine siskin
Red-breasted nuthatch
• Not seasonally or geographically
predictable
• Distances and number of migrants are
unpredictable
• Food specialists leave the boreal forests
for more southerly ranges when food
isn’t available
How do
we study
migration?
BANDING
US Fish and Wildlife Service Bird
Banding Laboratory
RADAR (NOAA)
Several years ago, researchers at
Cape May counted 14 million birds
in one night
RADIO TELEMETRY
Carl Safina followed a single radiotagged albatross throughout its
range
MOON WATCHING
Requires a full moon, a comfortable
chair and binoculars
How do
we study
migration?
CHRISTMAS BIRD COUNT
60000
52,471
52,471
50000
42,868
40000
32,322
30000
20000
15,000
10000
8,094
4,615
0
27
192
358
679
2,100
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Number of participants 1900-2000
THE BASICS OF FLIGHT
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
DEALING WITH DRAG
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
Eddy = DRAG
Airflow
Wing cross-section
Higher Pressure
LIFT
DEALING WITH DRAG
Wing
slotting
One way to reduce the drag caused
by these eddies is to break them up
into smaller eddies.
Primary feathers
create smaller
eddies = less drag
DEALING WITH 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.
THE BASICS OF FLIGHT
• Characterized by continuous flapping
Powered
• 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 climbing
and 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
Gliding birds keep their wings extended
and ride rising currents of warm air to
stay aloft. They flap only to regain altitude.
SAILPLANE 60:1
ALBATROSS 20:1
HAWK 10 to 13:1
MONARCH BUTTERFLY 3:1
Ratio measures horizontal distance to vertical drop
Route
finding
• Birds navigate by: visual landmarks, the sun, moon,
stars, and routes learned from other birds
• Birds also have an internal compass which is
sensitive to the Earth’s magnetic field
• Migrating birds will maintain a true compass
heading
STARS
MAGNETIC
NORTH
MOON
SUNRISE
WINDS
LANDSCAPE
FEATURES
WEATHER
UV LIGHT
SMELLS
SOUND
Birds usually select the most
efficient flight speeds.
How fast?
0
10
20
30
40
28-50
COMMON LOON
32-46
OSPREY
30-44
BROAD-WINGED HAWK
28-40
GOLDEN PLOVER
SEMI-PALMATED PLOVER
22-32
22-30
BALTIMORE ORIOLE
BLACKPOLL
WARBLER
50 mph
15-23
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
NORTH AMERICAN MIGRATION FLYWAYS
How far?
Atlantic Flyways
Mississippi Flyways
Central Flyways
Pacific Flyways
How far?
7,000 miles
one way
Red Knot
Flies from Argentina
to Brazil, to the
Delaware Bay,
to the Arctic.
How far?
Blackpoll warbler
Winter in South
America, and fly
to the Caribbean,
and then to northern
breeding grounds.
2,500 to
5,000 miles
one way
How far?
Some fly across the
Gulf of Mexico. (500
miles nonstop)
Hummingbirds
Winter in Central
America and fly as far
north as Canada.
How far?
7,000 miles
one way
Blue-winged Teal
Winters in South
America, and breeds
in northern plains
DAY OR NIGHT?
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 flight
are nocturnal migrants. Why…?
• Night flying helps birds avoid predators
• Other benefits include calmer, more
stable air
Fat is the
currency of
migration
• Fat is the most important fuel for
migration
• Many species double their weight with fat
fuel for migration
Typical Body Fat: PRE-MIGRATION
Shorebird
Songbird
66%
70%
Hawk
15%
Fat is the
currency of
migration
RED KNOT BODY FAT
3%
66%
PRE-MIGRATION
(Brazil)
POST-MIGRATION
(Delaware Bay)
Re-fueling
stations are
critical
When a Red Knot arrives at the Delaware Bay,
in two weeks it must increase its body weight
by 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 it
all into fat!
Birds rely on
the same
re-fueling
stations
Stopover places provide abundant food
for re-fueling needed by shorebirds to
complete migration, as well as roosting places.
James Bay,
Canada
Gray’s Harbor,
WA
Delaware
Bay, MD
Platte River,
NE
San Francisco
CA
Cheyenne
Bottoms,
KS
Copper River
Delta, AK
Bay of
Fundy, ME
The biggest threat to migration is habitat loss
Threats to
migration
• Loss of non-breeding ranges due to agriculture
and seaside development
• The destruction of the tropical and boreal forests
• Habitat fragmentation
Everything
in the world
is connected
to everything
else
• Migratory routes are the invisible lines of
connection which show us how people,
places, and wildlife depend on each other
• Migration tells us about our physical
and spiritual health
Sources &
further
reading
Ornithology
Frank B. Gill, W.H. Freeman & Co, 1995
The Random House Atlas of Bird Migration
Jonathan Elphick, ed., Random House, 1995
The Flight of the Red Knot
Brian Harrington, W.W. Norton & Company, 1996
How Birds Migrate
Paul Kerlinger, Stackpole Books, 1995
The Audubon Encyclopedia
of North American Birds
John K. Terres, Alfred A. Knopf, 1982
Living on the Wind
Scott Weidensaul, North Point Press, 1999
Connecting People with Nature