Home Adaptations Migration Hazards

Hazards

(Courtesy NEBRASKAland Magazine/NGPC)

Storms
Of all the hazards confronting birds during migration, storms are among the most dangerous. Birds that cross broad stretches of water can confront headwinds associated with a storm, become exhausted, and fall into the waves.

In fact, a catastrophe like this was once witnessed 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. They had accomplished nearly 95 percent of their long flight. But they were caught by a "norther" – a sudden, cold gale from the north. The birds could not make make headway against the storm. 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.

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Lighthouses, tall buildings, monuments, television towers, and other aerial obstructions have been responsible for destruction of migratory birds. It isn't just the constructions themselves – it also involves their lights. Bright beams of lights on buildings and airport ceilometers (devices that use a light source to measure the height of the cloud base) have a powerful attraction for nocturnal (nighttime) air travelers. It's similar to the fascination for lights exhibited by many insects, particularly night-flying moths. Their 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.

(Courtesy NEBRASKAland Magazine/NGPC)

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. A television tower's blinking lights can cause passing migrants to blunder into guy wires or into the tower itself. Many incidents like this have been reported throughout the United States, which shows this peril to migration is widespread.

Dehydration
It's clear from observation that under favorable conditions, birds can fly when and where they please. The distance covered in a single flight is governed chiefly by the rate of dehydration and, to a lesser degree, by the amount of stored fat available to the bird.

Wind Turbines
Birds sometimes fly into these very tall structures, and the noise made by the turbines can also affect birds.

Power Lines
Migratory birds run into problems with power lines because they sometimes don't see them or see them too late and end up crashing into them. Also, birds sometimes perch on the lines and get electrocuted. Usually, small birds are able to perch without harm because they don't complete a circuit (they don't touch a grounded wire or structure or touch another energized wire while sitting on the line). Bigger birds do tend to touch the power line and a ground wire, another energized wire, or a pole at the same time, and the result is electrocution.

There are some efforts underway to reduce these problems. Wires can be marked to make them easier to see. There are bird flappers and diverters that can be attached to wires to help birds notice them before it's too late to steer clear. To reduce electrocution risk, some power companies are insulating wires and increasing distance between wires to avoid deadly contact when birds perch.

Patterns
Migration patterns and routes are almost as varied as the migrant birds themselves. The main migration routes of North American land birds are oriented north-south, partly because the coasts, major mountain ranges, and major river valleys tend to run in the north-south direction.

(Courtesy NEBRASKAland Magazine/NGPC)

Although most birds make their migratory flights either by day or by night, there are birds that travel at both times. Because many species of wading and swimming birds are able to feed at all hours, they migrate either by day or by night. Some diving birds, including ducks that submerge when in danger, often travel over water by day and over land at night.

The Atlantic coast is a regular avenue of travel, and is well known for many famous locations for observing both land and water birds. About 50 different kinds of land birds that breed in New England follow the coast southward to Florida and travel from there by island and mainland to South America.

Another route is a direct line of travel for Atlantic coast migrants en route to South America, although it involves much longer flights. It's used almost entirely by land birds.

After taking off from the coast of Florida, migrants have only two intermediate land masses where they can pause for rest and food. Nevertheless, tens of thousands of birds representing about 60 species cross the 150 miles from Florida to Cuba, where many remain for the winter months.

The others negotiate the 90 miles between Cuba and Jamaica. From that point to the South American coast, there is a stretch of island-less ocean 500 miles across. The Bobolink outnumbers all other birds using this path. In fact, they're so much in the majority that this route could be designated the "Bobolink Route."