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2009-12-17 digital edition

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2009-12-17 / Community News

About Bird Migration

Bird migration has fascinated and confused humans for centuries. Noted scholars like Aristotle thought swallows hibernated in the mud during winter and reappeared during the spring after they thawed, and John James Audubon reported seeing a hummingbird riding on the back of a goose. Both of these men were wrong in these conclusions. Bird migration is an annual event that allows birds to take advantage of areas with seasonally high abundance of food and in turn avoid these areas when conditions are bad. Classical examples of migrants are geese that breed in the Arctic and winter on the Texas Coast when conditions are too severe in the Arctic.

Why do birds migrate? Many are believed to have developed migration patterns in response to changing climate conditions. Species living mostly in tropical settings adapted to long days and short summers available to nest in the north when glaciers retreated, and they returned to the tropics for the winter. The longer days of the northern summer provide greater opportunities for breeding birds to feed their young. The extended daylight hours allow diurnal birds to produce more eggs than related non-migratory species that remain in the tropics year-round. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.

How can they do this? Some species breeding and wintering grounds are separated by hundreds if not thousands of miles. Birds involved with long distance migration needed to develop super navigation abilities to be able to relocat their breeding and wintering grounds, an ability to rapidly store and utilize food as fuel, and a great ability for long distance flying.

Navigation abilities varies with the distances involved in migration and the time of day in which they migrate. Many species have developed abilities to use stars and sun as migration markers in their travels. Some species are able to detect magnetic fields within the earth to assist in migration, and others just use visible major landforms like mountains and rivers.

Traveling long distances usually require lots of energy. Most species build up large amounts of fat varying from 50 to 100% of their normal body weight in the days/weeks before major migration flights occurs. This fat is used as fuel and birds, unlike humans, can burn off all of their fat within 1 to 2 days of flying.

By migrating at night, small birds minimize predation, and avoid overheating that could result from the energy expended to fly such long distances. This also enables them to feed during the day and refuel for the night.

People interested in further reading on migration should look in their favorite library or bookstore for SCOTT WEIDENSAUL’S BOOK ON “Living On the Wind: Across the Hemisphere With Migratory Birds” that was published by Douglas & McIntyre in 1999.

If you would like to contact your local biologist, see our website at; http:// www.tpwd.state.tx.us/wildlifebiologist.

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