Natural Navigators Millions Of Birds Take Flight With The Change Of Seasons; Findng Their Destination With Uncanny Accuracy

In the third century B.C., Aristotle - philosopher, pupil of Plato and tutor to Alexander the Great - made an astute observation: Every fall in his native Greece the birds seemed to congregate and make an exodus en masse, returning only when spring had once more arrived.

Aristotle, known to modern biologists as the father of natural history, wondered what made some birds leave in the first place. Then he wondered where on earth they spent the winter.

He came up with the theory of transmutation. He figured the towhees hopping about his yard in the spring simply changed themselves - or transmuted - into the robins he saw in the winter.

Aristotle also theorized that the swallows he saw arriving in the spring had done as frogs do - hibernated over the winter in the mud. This theory doesn’t seem too far-fetched to those who have come upon groups of cliff swallows clamoring together in spring mud puddles. We now know that the birds are only scooping up beaks full of mud with which to construct nests.

Because he never travelled very far from Greece during his lifetime, Aristotle didn’t have the opportunity to see the great flocks of birds winging their way toward Africa. Had he journeyed to Israel in September or October, he might have witnessed the spectacular raptor migration that funnels annually through the Middle East.

Though his theories seem pretty ridiculous today, it is still significant that Aristotle provided the first account in written history of man pondering the act of migration.

Much of what we know about migration has been learned only in the last 50 years.

Migration is a term derived from the Latin word migrare, to go from one place to another. The act of migration refers to the seasonal movement of birds from their wintering areas in the fall to nesting areas in the spring and vice versa.

Birds migrate for two important reasons: food abundance and longer days in which to gather that food for nestlings.

Day length and air temperature are important cues for migrations to begin. In late summer, a bird’s physiology begins to undergo terrific changes. As the days shorten, the pituitary glands release hormones that cause birds to molt, or shed, their feathers.

On their breeding grounds, birds travel short distances for food and have no need to fly very far. Their new feathers are strong and resilient, custom-made for the long journey ahead.

Another rather drastic physical change involves a bird’s sexual organs, which begin to dry up and disappear!

After molting, migratory birds begin to put on a layer of subcutaneous fat, sometimes doubling their body weight. This accumulation of body fat will help provide energy for the long migratory flight. While the pituitary gland simultaneously controls sexual and migratory cycles, the urge to migrate depends on a number of endocrine glands as well.

Another important player in migration is the precession of the equinox, the time when the sun crosses the plane of Earth’s equator.

The autumnal equinox occurs about September 22, causing Earth to tip North America away from the sun as it tips South America toward the sun. That change alerts millions of birds that it is time to travel south for the winter.

In caged migratory birds, a decided restlessness becomes apparent about this time. Captive birds will begin to flutter against their cages in attempts to orient themselves in the right direction for migration. Studies have shown that in the spring, birds orient this fluttering behavior to the north. In the fall, birds try to orient themselves to the south.

We know that some birds travel thousands of miles to and from breeding and wintering grounds, but how do they know how to get there?

Part of the answer lies in genetics. Birds that have successfully migrated south and made it back to the north to breed and rear young pass on their genes to their broods. To put it simply, a good sense of direction is passed from one generation to the next.

Birders get a kick out of documenting bird species that are present outside their normal ranges, but often, these birds are doomed because they possess inferior genetic information. Granted, some birds are forced beyond normal ranges for reasons like a shortage of food, as may be evidenced by the periodic winter influx of snowy owls on the Rathdrum and Palouse prairies. However, a bird that goes off course during migration rarely has a chance to benefit from the mistake.

Scientists think some birds may undergo chemical changes in the brain that aid in migration. One friend of mine with an unusually good sense of direction jokes that he has “iron in his nose.” It may be that some bird species have magnetite, a form of iron oxide, concentrated in their brains that is tugged by Earth’s magnetic field, acting like a crude compass to guide them.

Only a portion of birds migrate by day. They may possess an interior sun compass by which they navigate. Also, very large objects, such as mountains or skyscrapers, may act as giant sun dials.

Big, broad-winged birds such as eagles and cranes tend to migrate during the day because they can use heat thermals rising off of the earth’s surface to buoy their flight.

Some birds have the capability to migrate during either the day or night. That group includes Canada geese, loons, and even blackbirds and the American robin.

Most of the smaller songbirds feed and rest during the day and migrate under the cover of darkness, allowing them to escape the notice of predatory birds such as hawks and gulls.

Studies have shown that birds can navigate by the stars, using the different constellations within a 30-degree circular area around the north star. Birds may be able to extrapolate direction and distance by using the rotation of the earth against the night sky, meaning birds can instinctively do their own form of geometry and algebra.

Many birds in Idaho and Washington don’t migrate at all, or migrate only short distances, say a few thousand feet up or down a mountain. Chickadees, pileated woodpeckers, and ruffed grouse are examples of birds that tend to stay close to home during the winter.

At least two thirds of birds that breed in the United States migrate. The most common destinations for migrating birds are between the southern U.S., Mexico and Central America. Some migrate to the West Indies or as far away as South America.

The Arctic tern makes perhaps the most grueling and spectacular migratory journey of all North American birds. From its nesting grounds in Greenland, the Arctic, and Alaska, Arctic terns fly more than 10,000 miles to winter in the Antarctic - an annual round-trip of more than 20,000 miles.

Birds also use weather patterns and geographic landmarks to orient themselves. Birds may use only one of the methods described, but more likely, they rely on a combination of methods to guide them on a successful migration.

This fall when you are snuggled deep within your flannel sheets and hear a flock of Canada geese hollering as they wing in midnight flight over your house, wish them Godspeed on their southward journey. It takes a miracle of nature to get them there.