Scientists ask: How dobirds know?

Imagine: Tiny birds leave their homes in the north and fly thousands of miles south to their winter homes. In spring, they fly thousands of miles north to their breeding grounds.

Even more amazing is the fact that so many species of migratory birds are very site-specific. That is, they return to the same place - often the same nest or forest clearing - year after year.

Birds migrate to find food, water, and shelter. The breeding grounds - summer homes - of many migratory birds are spread throughout the northern United States and Canada. Insects are abundant here in the summer. As the weather turns cold, the insects die or disappear. The birds - flycatchers, hummingbirds, warblers, vireos, and thrushes - fly to the tropics.

We like to think that "our" North American birds migrate to the tropics. But researchers say it's more likely that the birds' tropical ancestors left their tropical breeding grounds and flew north. The attraction: a more temperate zone (not as hot) with lots of food and many more hours of daylight in the summer. More daylight means more time to gather food.

The birds are "migrant workers that exploit the huge 'bloom' in insects in the north in the summer," says Kevin McGowan, an ornithologist at Cornell University in Ithaca, N.Y. "They're not 'our' birds taking vacations in the south; they're southern birds that come up here to take advantage of a temporary period of riches."

The birds prepare for their journeys by eating 25 to 30 percent more than usual to build up fat. The fat provides most of the "fuel" they need for the trip.

Migrating songbirds can fly as far as 620 miles without stopping. But they're more likely to make shorter flights of 200 miles or so. They must stop to eat and recover for a few days between flights. These rest stops along migration routes are very important.

We don't often see flocks of songbirds migrating, the way we see migrating geese in their V formations. That's because songbirds fly mostly at night. It's cooler then, so it's less stressful. And then they can spend their daylight hours looking for food.

How do these tiny birds know where to go? A first-year bird that's never flown south will head toward its species' wintering grounds seemingly without guidance from older birds. Many return to the same spot, year after year. Scientists have found that birds use the sun, stars, even the earth's magnetic field to navigate. But how songbirds return to specific spots on their breeding grounds or wintering grounds is truly a mystery.

(c) Copyright 2001. The Christian Science Publishing Society