Northern Prairie Wildlife Research Center
Migration of Birds
Perils of Migration
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.
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.
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.
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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