The stereotypical migratory bird moves roughly north-south, dividing its time between tropical winters, and summers in the higher latitudes. In general, migratory birds must fit into territory where there is not too much competition for resources. The summer months in the respective hemispheres provide greatly expanded potential range and food, which migratory birds exploit to their breeding advantage. For this reason, most migrants do travel roughly north-south.
Many birds, however, move only to the lower edge of their summer range in winter, travel east-west to a sea coast, or simply move to a lower altitude in the same place.
Migratory Birds are Mainly Northern Species
Most of the land on Earth is concentrated in the Northern Hemisphere, and because of this, the majority of migratory bird species travel north to breed and return south for the winter. Many seabirds do breed in the Southern Ocean-their migrations are primarily guided by powerful ocean winds and currents.
Migration Flyways
Birds may be seen migrating almost anywhere, but geographic features and food availability tend to funnel them along general routes called flyways. Mountain ranges, large bodies of water, river systems, ocean coastlines, and deserts create guides and obstacles that shape flyways and, to some extent, determine destinations.
North-South Migration
About five billion land birds follow north-south migration routes in the Americas, and even more do so in Europe, Asia, and Africa. They don't all leave the tropics and travel to the far north or Antarctica however. Many species move shorter distances from intermediate locations, while others move into vacated territories from further south. Within a single species, some populations may not move at all, some may move only a short distance, and others may pass over occupied territory to travel farther afield. Because seasons in the two hemispheres are opposite, a northward shift when the Northern Hemisphere moves into spring coincides with the return of southern breeders in their fall.
East-West Migration
Particularly in Europe and Asia, some birds follow an east-west migration route, which usually takes them inland during the summer, and back to an ocean coastline for winter. Weather is generally milder at the coast and the water remains open year round, so that sufficient food is available through the winter months.
Migration to a Different Altitude
Some bird species stay in the same general location all year round, but move to a higher elevation in the warmer months. Like north-south migration, this movement exploits territory and food supplies that are off limits in the winter.
Seabird Migration
Some seabirds, such as Arctic terns are champion migrants in terms of distance travelled each year. Some species have distinct wintering grounds, while others roam far out at sea outside the breeding season. Migration routes typically follow coasts, where water is shallow and sea life abundant, or ocean currents which carry food from deep ocean upwellings.
All Movement Isn't Migration
Sometimes large populations of birds move together in search of food. Emus in Australia do this, congregating in thousands and moving like nomads to greener pastures. Finch irruptions often involve mixed finch species vacating their range and moving elsewhere when seed crops are poor. These movements, dependant on conditions, are not predictable, and are not true migrations.
Related Content:
An Avian Migration Glossary
Birds Know When to Migrate
Sources:
Atlas of Bird Migration. Elphick, Jonathan ed. Buffalo: Firefly Books, 2007.
Firefly Encyclopedia of Birds. Perrins, Christopher ed. Buffalo: Firefly Books, 2003
Migration of Birds. Lincoln, Frederick C., Steven R. Peterson, and John L. Zimmerman. U.S. Department of the Interior, U.S. Fish and Wildlife Service, Washington, D.C. Circular 16. Jamestown, ND: Northern Prairie Wildlife Research Center Online. (Version 02APR2002), 1998.
http://www.suite101.com/content/bird-migration-patterns-a71534
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