Feathers are made of a strong, durable material (keratin), but eventually they simply wear out because of UV radiation, wind, and dust particles. Since worn-out feathers can't be repaired, they must be replaced.
Molting is the process of replacing old feathers with new feathers. As the new feathers begin to grow, they push upward on the old feathers, causing the old feathers to loosen and eventually fall out. It's similar to what happens as your baby teeth are replaced by the mature teeth underneath, but for birds this replacement process happens more than once in a lifetime.
The number of times molting occurs each year varies among birds. Molting is very expensive for a bird – it takes a great deal of energy to replace feathers.
All feathers are replaced at least once every year. The timing might depend on photoperiod (the amount of time per day the bird is exposed to light), mating, migration, and/or the availability of food. Feathers that cover the body (coverts) are typically replaced two times per year, while the flight feathers (rectrices and remiges) are replaced only once.
The Molting Sequence
Immediately after hatching, the birds begin to grow their next set of feathers, which is called juvenal plumage. Typically, a bird keeps this set of feathers (plumage) for the first 18 weeks of its life.
During the fall, the birds lose their juvenal plumage and grow basic plumage. All of the feathers are replaced during this molt, so the birds will have new feathers for surviving the winter. Birds will retain this basic plumage for most of the winter.
As spring approaches, the birds molt again – usually only the covert feathers this time. The bird's basic plumage is replaced with its alternate (mating) plumage. The alternate plumage is the plumage worn by birds during the mating season. Typically, the alternate plumage of males is much brighter and showier. This eye-catching display helps the male to attract a mate.
Birds can be seen in their alternate plumage until fall, when they molt back into their basic plumage. This continuous molting cycle – basic plumage in the fall and alternate plumage in the late winter/early springtime – continues for the rest of a bird's life.
For example, Bald Eagles may take four or five years to develop their adult plumage (the bald head).
Other exceptions to the typical molting pattern include the Western Meadowlark and the American Robin, which molt by wear. These birds grow brightly colored feathers with dull tips in the fall. By the time the bright colors are needed in the spring and summertime, the duller ends have worn away, leaving the bright colors for display.
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