Migration of Birds

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.

Migration is dangerous. Untold thousands of smaller migrants die each year from storms and attacks by predators. Indeed, the passage of migrants is so reliable that Eleonora's Falcon breeds in the fall to take advantage of the many songbirds crossing the Mediterranean as a source of food for its young. Mortality during migratory flight, of course, is one of the several costs that balance the increased production of offspring that migrants obtain by nesting in locations where food is more abundant and interspecific competition for most resources is lower.

Storms
Of all the hazards confronting birds in migration, storms are one of the most dangerous. Birds that cross broad stretches of water can confront headwinds associated with a storm, become exhausted, and fall into the waves. Such a catastrophe was once seen from the deck of a vessel in the Gulf of Mexico, 30 miles off the mouth of the Mississippi River. Great numbers of migrating birds, chiefly warblers, were nearing land after having accomplished nearly 95 percent of their long flight when, caught by a "norther" against which they were unable to make headway, hundreds were forced into the waters of the Gulf and drowned. A sudden drop in temperature accompanied by a snowfall can cause a similar effect.

Aerial Obstructions
Lighthouses, tall buildings, monuments, television towers, and other aerial obstructions have been responsible for destruction of migratory birds. Bright beams of lights on buildings and airport ceilometers have a powerful attraction for nocturnal air travelers that may be likened to the fascination for lights exhibited by many insects, particularly night-flying moths. The attraction is most noticeable on foggy nights when the rays have a dazzling effect that not only lures the birds but confuses them and causes their death by collision against high structures. The fixed, white, stationary light located 180 feet above sea level at Ponce de Leon Inlet (formerly Mosquito Inlet), Florida, has caused great destruction of bird life even though the lens is shielded by wire netting. Two other lighthouses at the southern end of Florida, Sombrero Key and Fowey Rocks, have been the cause of a great number of bird tragedies, while heavy mortality has been noted also at some of the lights on the Great Lakes and on the coast of Quebec. Fixed white lights seem to be most attractive to birds; lighthouses equipped with flashing or red lights do not have the same attraction.

For many years in Washington, D.C., the illuminated Washington Monument, towering more than 555 feet into the air, caused destruction of large numbers of small birds. Batteries of brilliant floodlights grouped on all four sides about the base illuminate the Monument so brilliantly, airplane pilots noticed that it could be seen for 40 miles on a clear night. On dark nights with gusty, northerly winds, nocturnal migrants seem to fly at lower altitudes and are attracted to the Monument. As they mill about the shaft, they are dashed against it by eddies of wind, and hundreds have been killed in a single night.

In September 1948, bird students were startled by news of the wholesale destruction of Common Yellowthroats, American Redstarts, Ovenbirds, and others against the 1,250 foot Empire State Building in New York City, the 491 foot Philadelphia Saving Fund Society Building in Philadelphia, and the 450 foot WBAL radio tower in Baltimore. In New York, the birds continued to crash into the Empire State Building for 6 hours.

More recently, television towers have become a major hazard. These structures are so tall, sometimes over 1,000 feet, they present a greater menace than buildings or lighthouses. Their blinking lights cause passing migrants to blunder into guy wires or the tower itself. Numerous instances throughout the United States indicate this peril to migration is widespread. Yet TV tower kills have been an excellent source of scientific information on the fat loads migrants carry, since they literally remove birds from out of the sky during their migration.

Exhaustion
The American Golden-Plover travels over a 2,400-mile oceanic route from Nova Scotia to South America in about 48 hours of continuous flight. This is accomplished with the consumption of less than 2 ounces of body fat. In contrast, to be just as efficient in operation, a 1,000-pound airplane would consume only a single pint of fuel in a 20-mile flight rather than the gallon usually required. Similarly, the tiny Ruby-throated Hummingbird weighing approximately 4 grams, crosses the Gulf of Mexico in a single flight of more than 500 miles while consuming less than 1 gram of fat.

One might expect the exertion required for long migratory flights would result in arrival of migrants at their destination near a state of exhaustion. This is usually not the case. Birds that have recently arrived from a protracted flight over land or sea sometimes show evidences of being tired, but their condition is far from being in a state of exhaustion, unless they have faced adverse winds. In reality, even small landbirds are so little exhausted by ocean voyages, they not only cross the Gulf of Mexico at its widest point but may even proceed without pause many miles inland before stopping. The Sora, considered such a weak flyer that at least one writer was led to infer most of its migration was made on foot, has one of the longest migration routes of any member of the rail family and even crosses the wide reaches of the Caribbean Sea. Observations indicate that under favorable conditions birds can fly when and where they please and the distance covered in a single flight is governed chiefly by the rate of dehydration and to a lesser degree, the amount of stored fat.

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