Did You Know? Bird Migration

1. Bird Migration

There are about 10,000 bird species in the world and about half of them migrate. That’s an estimated 50 billion migratory birds out of the 200-400 billion individual birds on the planet. In eastern North America over two-thirds of all breeding birds migrate. They travel by day and by night across vast stretches of land and sea. They navigate using familiar landscapes, mental star charts, magnetic fields, and angles of ultra-violet light emitted by the setting sun that are invisible to us. Worshiped as gods by some past cultures, written into poetry, painted on canvas, studied by scientists, and loved by birders, bird migration is a marvel. From warblers to waterfowl, hummingbirds to hawks – each is a remarkable story. This fall, climb a hill and watch the hawks glide southward one sunny day. Or just before bedtime in a quiet place, go out and listen toward the sky. If you listen closely, you might hear the waves of songbirds overhead calling to each other as they head south for the winter.


Red Knot bird migration

Red Knot adult, nonbreeding © Rick and Nora Bowers/VIREO

2. Traveling Knot

Of the six subspecies of Red Knots found worldwide, rufa migrates the farthest.  It flies over 20,000 miles round trip from its breeding grounds in the Canadian arctic along the eastern seaboard of North America and down to its wintering area at the tip of South America. But these birds are in trouble. Over the past decade, aerial counts of the rufa population in Tierra del Fuego went from more than 56,000 to just 10,000 birds. Some biologists believe that without intervention, they may go extinct.


Blackpoll Warbler migration

Blackpoll Warbler adult male, nonbreeding © Rob Curtis/VIREO

3. Blackpoll Migration

With the coming of frost, Blackpolls congregate along the Northeast seaboard to feast. This tiny songbird’s migration is fueled entirely by stored fat. One evening, when the winds are just right, they launch into the night sky on a non-stop flight to the Caribbean or even their final destination, South America. Scientists have calculated that a blackpoll has to weigh more than seven-tenths of a gram to have enough stored energy to safely complete its flight. For us, the metabolic equivalent would be to run four-minute miles for 80 hours, according to ornithologists Tim and Janet Williams. They also found that if blackpolls were burning gasoline instead of body fat, they would be getting 720,000 miles to the gallon.


Broad-winged  Hawk migration

Broad-winged Hawk adult © Lloyd Spitalnik/VIREO

4. Gliding Home

While songbirds rely on body fat to fuel migration, the Broad-winged Hawk simply relies on the power of the sun. The broad-wing doesn’t gorge itself. It doesn’t have to, because it wastes little energy on flapping its wings. This hawk, like many hawks, is a glider. Using the lift from rising columns of hot air, hundreds, thousands, or even tens of thousands of hawks congregate in flocks, or “kettles,” slowly circling upward on thermals and then gliding southward and downward to catch the next free ride. They rarely, if ever, sail over open water where few thermals exist. They follow the land as it becomes narrower and narrower through Central America, until some of them make it all the way into South America after a month of migration.


Arctic Tern migration

Arctic Tern adult © Arthur Morris/VIREO

5. A Bird of the Poles

Arctic Terns may have the longest annual migration of any animal in the world. Their lifetime frequent flier miles would be equivalent to three round-trips to the moon. Scientists recently used a tiny new device attached to the terns called a geolocator. It regularly records light levels and time of day, which can be used to generate its latitude and longitude, much like mariners once did. They found that the Arctic Tern averages 44,000 miles round-trip each year from Greenland to the shores of Antarctica and back.


Whooper swan migration

Whooper Swan adults © Jari Peltomaki/VIREO

6. Thin Air

Most migrant birds flying over lowlands reach altitudes of around 2,000 feet in North America. Raptors migrating over Texas have been found to be as high as 4,000 feet. Long-distant travelers over the ocean have been found as high as 20,000 feet. A pilot once reported Whooper Swans at 27,000 feet. But the records go to species in other parts of the world. Bar-headed Geese, a species regularly found at zoos, climb to over 30,000 feet when passing over the Himalayan Mountains. The all-time record belongs to Ruppell’s Griffon Vulture flying over Ivory Coast at nearly 38,000 feet when it was unfortunately sucked into a jet engine.


Bar-tailed Godwit migration

Bar-tailed Godwit adult, nonbreeding © Steve Young/VIREO

7. Godspeed Godwits

Using satellite transmitters scientists found that Alaskan Bar-tailed Godwits make an 8-day, 6,835 mile autumn migration from Alaska to New Zealand in one, long flight. This is almost double what scientists thought birds were capable of physiologically. They expend energy up to 10 times the rate they would at rest, besting riders of the Tour de France who manage just a six-fold increase. By the end of the journey, some males may have lost so much weight that their satellite transmitter backpacks simply fell off.


Swainson's Thrush migration

Swainson’s Thrush adult Olive-backed © Brian E. Small/VIREO

8. Not a Silent Night Flight

On an autumn night after the passage of a cold front millions of songbirds are passing overhead as we sleep. Waves of birds take to the skies even lighting up otherwise quiet weather radar. The birds are not silently floating over us. Especially in eastern North America, on a good night of migration listeners can here hundreds, even thousands of birds calling as they pass overhead. Some species are readily identified by their familiar call, like Swainson’s Thrushes. No one knows why they call as they travel. It may to keep in contact with neighbors to avoid collisions.


snow geese migration

Snow Geese adults © Arthur Morris/VIREO

 9. A Skein of Birds

Have you ever wondered why some bird flocks fly in a “V” formation? First, it is to conserve energy by taking advantage of the upwash vortex fields created by the wings of the birds in front. The rising air helps lessen the load. Each bird can reduce drag by up to 65%, increasing their range by 71%. Of course the leader of the formation gains no benefit, causing this position to be regularly traded. The other reason is to facilitate orientation and communication among individual birds. There are fewer blind spots in formation.


Snowy Owl migration

Snowy Owl juvenile © Scott Linstead/VIREO

10. South is in the eye of the beholder

The classic theory held by ornithologists to explain the irregular migrations of Snowy Owls southward has always centered on lemmings, a favorite food of the arctic breeding owls. These small rodents undergo population booms and busts. About every four years lemmings become incredibly abundant. Many scientists believed that after the lemming populations eventually crashed, snowy owls would head southward in search of food. Intensive research now shows this may not be the case. We find the highest numbers of owls south of the arctic in winter during lemming population booms, not after the lemming population has plunged. Most of them are juvenile birds that may have been pushed southward by the adults.


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