Birds face three major energetic challenges over the course of a year. Nesting and all its associated behaviors dominate spring and early summer.
Parents must build nests, defend territories, and feed their constantly hungry broods.
Migratory species travel great distances, often over open oceans where treacherous weather is an ever-present threat.
Molt
And then there’s the annual process of feather replacement. Ornithologists call feather replacement “molt.”
After 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. 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.
If you’ve ever found a single tattered feather on the ground in your backyard, it almost certainly got there as a result of molt. (Find a pile of feathers, and it’s probably the result of a predator attack.)
Fall molt is most obvious among backyard goldfinches. All summer long, males have been dressed in brilliant yellow, white and black feathers.
Color
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.
Their transformation from colorful feathered jewels into inconspicuous drab little birds is so complete many casual backyard birders fail to recognize them.
I get letters every fall from readers who ask me to identify a bird they describe perfectly as a goldfinch in winter plumage.
It will be late April or May when these familiar feeder birds again brighten our lives. Many birds molt just once each year, after the breeding season.
Replacing feathers, which make up 4 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 breed (or migrate) at the same time. 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 ensures that the wings and tail remain symmetrical, so the bird can still fly.
Some birds have a finely tuned molting pattern that occurs while they nest. But this requires, among other things, a very attentive mate.
Female ospreys, for example, molt on the nest. The male brings her food while she incubates the eggs and broods the young.
By the time the young ospreys leave the nest, mom has a new set of feathers. The male, on the other hand, follows the more conventional pattern and molts after nesting.
Water birds
Many water birds exhibit an entirely different “simultaneous” molt. 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 birds 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.
This is the pattern goldfinches, warblers and many other songbirds follow.
Not all birds molt twice a year. Some (geese and swans, for example) molt just once a year, others (ptarmigans) three times each year, while still others (eagles and vultures) may require several years to completely replace a set of feathers.
The only general rule that applies to molt is that it is a wonderfully flexible form of growth that adapts well to almost any environmental circumstance.
Changing day length, nature’s most reliable environmental cue, triggers internal hormonal responses that stimulate molt.
Because breeding and migration are also tied to photoperiod, the precise synchronization of all three phenomena is one of nature’s more amazing achievements.
