Thursday, October 8, 2009

Bird Migration

By Susan Benson,
CNHM Director of Education

It is estimated that as many as five billion birds migrate through the Americas every year. Approximately two-thirds of all North American bird species migrate. This means they travel from a breeding area to a wintering area and back again. They eat more food to gain fat for the flight. They molt old feathers, growing new ones. Many of these birds travel thousands of miles. They have no GPS or maps to navigate through the sky. The migration is timed so that young hatch as spring and fall berries and seeds ripen and insects arrive or decline. Finally, think of the ruby-throated hummingbird alone, whose weight is that of a penny, is about three inches in length, and makes a flight trek of thousands of miles while keeping a course through winds and weather. Bird migration is truly one of nature’s annual wonders.

Most scientists believe that birds navigate using the sun or stars as a compass. Some believe there is a chemical, a molecule called a superoxide, in birds’ eyes that allows them to sense the Earth’s magnetic lines as if there were a highway through the sky. Others believe the iron-rich magnetite crystals found in many birds’ brains allow them to detect the magnetic fields. Using visual landmarks, making mental maps, using olfactory cues, or genetic or environmental influences may also be amazing factors that help birds succeed with navigation and orientation.

Migrating birds generally move from north to south and from south to north. However, there are several birds that migrate over regular routes diagonally or even east to west before arrive at their destination. These diagonal travelers usually move only to the lower edge of their summer range in winter, travel east-west to a sea coast, or simply move to a lower altitude in the same place.

In late summer, before migration, the metabolism of migratory birds undergoes extreme changes. The actions of the hormones prolactin and corticosterone cause migratory birds to accumulate large amounts of fat under the skin. These accumulations provide energy for long flights.

Some birds migrate during the night to avoid predators and to eat and rest during the day. Nocturnal migrants will utter sharp and melodious peeping and piping calls, allowing individuals to stay in touch with each other.

Each bird creates behind it a small area of disturbed air during flight. Birds have learned to use this air to their advantage during migration. Air lost over the wing tips creates a spiral vortex behind each wing tip, with upswelling air on the outer side of each wing. Birds that migrate together fly aside or slightly above the bird in front and use less energy.

Migration is a very strenuous activity that requires extreme amounts of energy. Some birds will fly four to six days straight without any rest. Some researchers estimate that an equivalent feat would be similar for a human to run four-minute miles for a total of eighty hours!

We may never discover the scientific mysteries of the phenomenon of this annual cycle. Keeping a bird journal can help us learn a little more about our feathered friends. Record observations on a calendar and jot down some notes. Include any other observations of these birds. Perhaps you will discover a clue to solve the mystery of migration.

For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility and exhibits, the Curiosity Center and Brain Teasers 2, in Cable at 13470

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