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Migration of Birds

Influence of Topography

The relation of the world's land masses to each other and the distribution of ecological communities within these land masses influence the direction birds migrate. Topography may aid, hinder, or prevent the progress of a migrant depending on the bird's particular requirements. Old World migrants must contend with east and west trending mountain ranges and deserts, and major migratory routes tend to be in a northeast to southwest or a northwest to southeast direction in order to circumnavigate these barriers. In the New World, however, migrants can proceed north and south across the land unimpeded, since the major mountain ranges and river systems are oriented in the same direction as the birds' migration.

Distinct features in the landscape, including rivers, mountain ridges, desert rims, or peninsulas appear to influence migratory travel by providing a landscape reference for orientation, especially when it is necessary to compensate for wind drift.

Large bodies of water constitute real barriers to soaring birds dependent on thermals, since water temperatures are usually less than land temperatures during both vernal and autumnal migratory periods and thus are characterized by subsidence of air rather than updrafts. The shoreline, then, may provide a guiding line, since onshore winds rise upward once they move across the warmer land. These conditions often concentrate Broad-winged, Rough-legged, Red-shouldered, and Red-tailed hawks migrating through the Great Lakes into restricted areas where numbers observed can be spectacular. It has been observed around Lake Ontario, for example, that maximum hawk flights occur when winds are from 10 to 25 miles per hour, but when winds exceed 35 miles per hour good soaring conditions are curtailed and hawk migration ceases. Similar conditions exist over the Bosphorus between the Black Sea and the Mediterranean where thousands of White Storks, eagles, and buzzards can be observed on a good day. For migrants not dependent upon soaring flight, on the other hand, large bodies of water do not affect their rate of migration or the routes they choose. The Gulf of Mexico, the Mediterranean Sea, and even the open Atlantic from the Maritime Provinces of Canada to the northern coast of South America are regularly crossed by many songbirds.

As previously noted, mountain ridges that parallel the line of flight offer updrafts to soaring birds. The highest and longest ridges deflect the horizontal winds upward better than the shorter ridges less than 1,000 feet high, and more birds are seen, on the average, along these higher ridges.

In summary, topography may help or deter a migrant in its passage. It affects different birds in different ways. In North America, migratory movements are continent wide, and no evidence indicates any particular part of the landscape influences all birds in the same manner. Certain bird populations may use regular geographic routes during migration, but they are usually not rigidly restricted to them because of topography.

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