Bird Migration Facts
by Kerry Scanlan, Vicki Piaskowski, Michelle Jacobi and Steve Mahler
WHAT IS MIGRATION?
Migration is the seasonal movement of birds, generally between breeding and non-breeding areas.
WHY DO BIRDS MIGRATE?
Food: The change of seasons causes a change in food supply, causing birds to move to an area with a more plentiful food supply.
Reproduction: Birds also migrate to a specific area to breed and raise their young.
Many birds that breed in North America migrate to areas south of the Tropic of Cancer (southern Mexico, Central and South America and the Lesser and Greater Antilles in the Caribbean Sea) in the fall (August-October) because of a decrease in their food supply. Many of these birds are insectivores; they eat mainly insects. (Most insects do not survive the North American winters except in larval or egg forms.) These birds remain on their non-breeding (wintering) grounds until April. Then in spring they migrate back to their breeding grounds in North America to take advantage of the plentiful insect food supply to breed and raise young.
These birds that migrate south of the Tropic of Cancer are called Neotropical migrants. A more correct term now used is Nearctic migrants. Nearctic is a word that refers to the Arctic as well as the temperate parts of North America. Since these birds spend more time in the tropics than on their North American breeding grounds, they would be migrating into the Nearctic region. Thus, they would be Nearctic migrants. They actually may be tropical birds that have learned to fly north to exploit the plentiful insect food resources there.
HOW DO BIRDS PREPARE FOR MIGRATION?
To prepare for migration, birds become hyperphagic. That means they eat more food, which is stored as fat for their long journey. Fat is normally 3% to 5% of the bird's mass. Some migrants almost double their body weights by storing fat before migration. The ruby-throated hummingbird weighs only 4.8 grams and can use stored fat to fuel a non-stop, 24-hour flight across a 600-mile stretch of open water from the U.S. Gulf coast to the Yucatan Peninsula of Mexico!
WHEN DO THEY MIGRATE?
- During the day:
- Many soaring birds, such as hawks, migrate by day. They travel inland by flying and catching thermals that occur only over land. (Thermal updrafts are rising columns of warm air that spiral upward and lift the birds up so they can fly without flapping, saving energy. Raptors also use thermals when they are not migrating.) Hawks and other raptors do not like to migrate over water. When they reach Mexico and Central America, where the land narrows between the Pacific Ocean and the Caribbean Sea, the hawks are funneled over this land bridge. As so many birds try to stay inland, you will see huge concentrations of raptors, sometimes as many as 100,000 in one day.
- Insectivores, such as swifts and swallows, also fly during the day, feeding on insects as they fly.
- Flocking birds such as waterfowl and some finches fly during the day, too.
- During the night:
- Most songbirds travel at night. They spend the daylight hours resting and searching for food in the unfamiliar places where they stop to rest. It is thought that the lower night temperatures and stiller air make better flying conditions.
HOW FAR DO BIRDS MIGRATE?
It depends on the bird species.
The arctic tern may hold the record for longest migration distance since it flies about 30,000 km (18,600 miles) each year traveling between its arctic breeding ground and non-breeding area in the Antarctic. This amazing feat is possible because terns eat fish and can feed during their long journey.
Most songbirds don't fly to their non-breeding grounds non-stop. They stop a number of times to rest and feed during migration. The places they stop are called stopover sites, or staging areas. Birds remain at stopover sites for varying amounts of time based on the weather and how much fat they have stored. Some birds stop only one day to rest and feed, and then continue their migration. Others will remain at stopover areas for weeks. Most Neotropical migrants stop along the way to rest and feed.
Some birds are short-distance migrants and migrate only as far as they need to find food such as insects, seeds and berries.
Some birds are austral (southern) migrants. In the tropics, they migrate north to breed, then head south at the end of the breeding season. In Belize, they also are called dry-season residents because they migrate north to breed during Belize's dry season.
AT WHAT ALTITUDES DO BIRDS MIGRATE?
Some geese and ducks fly at incredible heights. Bar-headed geese have been recorded as high as 29,000 feet when they migrate over the Himalayas! That's five miles above our heads, even higher than Mount Everest!
Most night-migrating songbirds fly below 2000 feet (600 m) when flying over land. Some will fly as high as 6,500 feet (1,980 m). Occasionally, they may fly higher to reach favorable winds.
The wind sometimes causes birds to fly at certain heights. When the bird is flying into the wind (called a headwind), it flies very low. When the wind is blowing the same direction as the bird, pushing it along (called a tailwind), it will fly high, where the wind is the fastest.
HOW FAST DO BIRDS FLY?
In still air, most songbirds fly at 20 to 30 mph. Waterfowl and shorebirds can fly at 30 to 50 mph. A tailwind allows the bird to fly faster.
HOW DO BIRDS NAVIGATE?
Birds have excellent vision and rely on visual landmarks for local and long-distance migration. They use key land features such as mountains, rivers, coasts or even large buildings.
There are three types of "compasses" a bird uses to find its way. Birds can use the sun, the stars and the Earth's magnetic field.
- Birds use the sun as a compass. They use the positions of the sun during the day to navigate. They also can use the setting sun as an indication of due west.
- Night flyers use celestial navigation, which means they find their way by knowing the patterns of the stars in the sky, and by knowing special stars like the North Star. In their first year of life, birds memorize the position of the constellations in relation to the North Star. These star patterns stay the same even though the Earth moves through space, making the constellations appear to move to different spots in the sky during the year.
- Birds have tiny grains of a mineral called magnetite just above their nostrils. This mineral may help them to navigate using the Earth's magnetic field, which tells the bird what direction is true north.
Petrels and pigeons can use their sense of smell to find their way, but it is used only in addition to the sun, stars and magnetic field.
HOW DO WE LEARN ABOUT MIGRATION?
Scientists throughout the world conduct many types of research to learn about migration. Heavy concentrations of migrating birds can be seen on weather radar screens. Many bird observatories conduct migration counts to learn about the numbers and species of birds that migrate each year. Bird-banding research has allowed scientists to learn about migration. Scientists band many birds every year and sometimes those birds are caught again, or found after they die. By checking the band number and reporting it to the Bird Banding Laboratory, scientists can learn where the bird was first banded and how far it traveled.
Able, K.P. 1999. A Gathering of Angels. Comstock Books. Cornell University Press, Ithaca, N.Y.
Gill, F. B. 1990. Ornithology. W.H. Freeman and Co., New York, N.Y.
Smithsonian Migratory Bird Center. Smithsonian National Zoological Park, Washington D.C. Have Wings Will Travel: Avian Adaptations to Migration. (Fact Sheet by Mary Deinlein). http://nationalzoo.si.edu/ConservationAndScience/MigratoryBirds/Fact_Sheets/default.cfm?fxsht=4. (22 Jan. 2001).
Suggested reading for further information:
Other Fact Sheets from The Smithsonian Migratory Bird Center are great sources of information: http://nationalzoo.si.edu/ConservationAndScience/MigratoryBirds/default.cfm.
Weidensaul, S. (1999) Living on the Wind. North Point Press. (Division of Farrar, Straus and Giroux), New York, N.Y.
Oram, Liz and R. Robin Baker. 1992. Bird Migration. Steck-Vaughn Co., Austin, TX.
Sanders, John. 1984. All About Animal Migrations. Troll Associates, Mahwah, New Jersey.
Elphick, Johnathan. 1995. Atlas of Bird Migration. Random House, New York, NY.