By Scott Shalaway
Shorter days, a predictable characteristic of September, is the absolutely reliable environmental cue that stimulates birds to migrate.
Nothing heralds the coming of autumn like the sight of the distinctive V-shaped formation of a flock of migrating geese or a kettle of broad-winged hawks sailing south along an Appalachian ridge. But most song birds migrate unseen at night.
Though night migrants can't be seen, they can be detected in two subtle ways. On clear nights, watch the moon with a spotting scope or binoculars. A scope works better because it's difficult to steady binoculars for extended periods of time. But if you can maintain your focus for 20 to 30 minutes, you just might catch silhouettes of migrating birds crossing the path of the moon. It's an impressive, memorable and moving sight.
And keep your ears tuned to the night skies. Many song birds vocalize as they migrate, presumably to keep in touch with other members of the southbound flock. Ornithologists suggest that nocturnal flight calls help migrating flocks organize their spacing and avoid collisions. Identifying migrating birds by their flight calls is best left to experts, but anyone can hear sounds originating in the night or pre-dawn skies. (For more information about flight calls, visit www.birds.cornell.edu/birdcalls.)
According to the Cornell Lab of Ornithology, on a good migration night in eastern North America, thousands of calls may be heard by listeners on the ground. Migrating at night offers several advantages. Night flight permits celestial navigation, night skies offer more stable atmospheric conditions, and few predators patrol the night sky.
Birds migrate for a number of reasons -- to escape foul weather, to find a place to nest, or to find a dependable food supply. The mystery of migration is not so much why birds migrate, but how.
Blackpoll warblers, for example, fly from Maine to South America across the open Atlantic Ocean. Bristle-thighed curlews nest on Alaska's west coast and winter in Polynesia, a distance of about 6,000 miles. After nesting off the southern coast of Australia, short-tailed shearwaters circle the Pacific Ocean before returning the following year. They travel north along the Asian coast, head east across the Bering Sea, then south along the coasts of Alaska and British Columbia before striking out across the Pacific and back to Australia. The journey covers 20,000 miles.
But the grand champion migrant is the arctic tern.
Individuals that nest in Greenland and the eastern Canadian arctic travel south along the coasts of Great Britain, Spain and west Africa, then wing their way west and south across the Atlantic Ocean to Antarctica for the austral summer. When the Antarctic weather begins to deteriorate, they complete the circuit by heading north along the coasts of South and North America to their arctic breeding grounds. The total length of this incredible annual round trip measures about 22,000 miles.
Birds make navigation look simple. Displaced homing pigeons return to their loft with ease. In a classic experiment, a Manx shearwater removed from its burrow in Wales and flown more than 3,000 miles across the Atlantic Ocean to Boston returned to its nesting burrow in less than 13 days.
Birds apparently possess an internal map and compass mechanism that enables them to traverse unfamiliar areas and return home. So far no one understands the internal map -- how they know where they are going (though with experience they certainly recognize major land forms). But scientists have identified some parts of the navigational system of birds -- the internal compass.
Much of the research on bird navigation has been done with homing pigeons. They use the sun's position in the sky as a compass. As the sun's position changes throughout the day, pigeons automatically compensate for these changes.
Surprisingly, pigeons navigate equally well on cloudy days. They apparently possess a backup system that may be sensitive to the earth's north-south geomagnetic field. And night migrants, the ones you might see flying by a brightly shining moon, can navigate by the stars -- as long as the sky is clear.