A feather is a "dead" structure, somewhat analogous to hair or nails in humans. The hardness of a feather is caused by the formation of the protein keratin. Since feathers cannot heal themselves when damaged, they have to be completely replaced. The replacement of all or part of the feathers is called a molt. Molts produce feathers that match the age and sex of the bird, and sometimes the season.
Molting occurs in response to a mixture of hormonal changes brought about by seasonal changes. The entire process is complex and many questions remain regarding how the process is controlled. A basic understanding of molting patterns can, however, be a useful aid in identifying many species and in determining their age.
There are two kinds of molts with different degrees of feather replacement.
In a complete molt all feathers are replaced.
In a partial molt only some feathers are replaced
Damaged feathers are replaced during a molt. A feather that has been lost completely is replaced immediately.
It takes a lot of energy to build new feathers. Molting is, therefore, often timed to coincide with periods of less strenuous demands, such as during or after nesting. Barn Owl, molting of wing feathers is from the inside out and are replaced from the middle of the wing out (in both directions). Tail feathers also drop out a few at a time,
Molt Cycles:
How often do birds molt? This varies by species, but almost all birds fall into one of three categories.
For Barn owl its one molt per year, hawks as well fall into that category.
When it comes to molting, there are a lot of "except for the exceptions." The best resource for details on individual species can be found at the Cornell Lab of Ornithology's Birds of North America web site.
Note that molting periods cover a range as long as 3 months, and are approximations only. Some birds will start molting earlier and others later. All feathers are not lost at once during the molt, so there may be a high degree of variability as they molt from one plumage to the next.
