We call it autumn or fall, but we could call the upcoming season of transitions “M&M”-for bird “Migration and Molt.”
Late summer and fall’s shorter days are the only absolutely reliable environmental cue that alerts birds that it’s time to migrate and molt.
Though many songbirds migrate at night, they can be detected in two subtle ways. On clear nights, watch the moon with a spotting scope or binoculars. You just might catch silhouettes of migrating birds crossing the illuminated moon.
Nocturnal migrants can also be heard as they migrate. These “flight calls” enable birds to keep in touch with other members of the southbound flock. Night flight permits celestial navigation, night skies offer more stable atmospheric conditions, and fewer 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, however, is not so much why birds migrate, but how.
In a classic experiment, a Manx shearwater removed from its burrow in Wales and flown over 3,000 miles across the Atlantic Ocean in a plane to Boston returned to its nesting burrow in fewer than 13 days.
Birds apparently possess an internal map and compass mechanism that enables distant travel. So far understanding the internal map – how they know where they are going – remains elusive. But scientists have identified some parts of the navigational system of birds – the internal compass.
Pigeons, for example, 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 nocturnal migrants, the ones you might hear flying overhead at night, can navigate by the stars – when the sky is clear.
The other important transition that occurs in late summer and fall is molt, the process off feather replacement. Next to nesting, it’s the most important thing birds do. Feathers are critical for survival. They are essential for flight, and they streamline, protect, and insulate the body.
Fall molt is most obvious among backyard goldfinches. All summer long, males have been dressed in brilliant yellow, white and black feathers. A few still show splashes of gold, but most have begun to lose their luster. By Thanksgiving, many casual backyard birders won’t even recognize these drab little birds as goldfinches.
Molt is simply the process of replacing worn feathers. Feathers are made of tough material similar to human fingernails and hair. But feathers wear out. The constant beating feathers take from the elements exacts a steady toll. Birds molt to counter this relentless wear.
Many birds molt just once each year, after the breeding season but before migration. Replacing feathers, which make up four to 15 percent of a bird’s body weight, requires a lot of energy. Timing is critical. Most birds simply can’t consume enough calories to molt and nest (or migrate) simultaneously.
Wing and tail feathers are usually replaced during the late summer molt. These feathers are lost and replaced a pair or two at a time, one from each side. This insures that the wings and tail remain balanced so the bird can still fly.
Some birds have a finely tuned molting pattern that occurs while they nest. The female molts while on the nest, and her mate sees that she is well fed. The male, on the other hand, follows the more conventional pattern and molts after nesting.
Many water birds exhibit an entirely different molt pattern. Ducks, geese, swans, loon, grebes, and many rails lose all their flight feathers (wing and tail) at once. This renders them flightless for four to 10 weeks, depending on the species. Water birds survive during this flightless period by living on a pond or lake that provides open water for feeding and nearby dense vegetation for escape cover.
Many songbirds supplement the late summer molt with a spring molt of just body feathers. This enables them to assume their bright breeding colors without the energetic stress of a complete body molt.