Feathers get heavy use. As a result, they wear out. They cannot repair themselves when they have gotten worn or broken, the way our skin can when it has been injured, so they need to be replaced. This process of losing feathers and growing new ones is called molting. New feathers begin to grow into the sockets, and push out the old feathers, much as baby teeth are pushed out by incoming new teeth. Many birds molt twice a year - once in the fall to replace their worn feathers, and again in the spring into their breeding plumage.
Birds donít lose their feathers all at once, just a few at a time. The entire process usually takes from one to three months; songbird molt starts at the head and moves down to the tail. Molting usually takes place in the fall; this is the best time since the work of raising young is finished, and by the time the bird is ready to migrate, it has new, healthy feathers for the long journey. Molting also taxes a birdís energy. It is a good idea to hang a cake of suet in the fall for these birds.
Take a look at this Blue Jay, which has lost some of the feathers on its head: Blue Jay.
Birds are more vulnerable during their molt, especially if they have lost some flight feathers. They become shy and wary. There may be days when you wonder where all the birds are gone. They are still around, just hiding. Look at this warbler which has lost one of its primary flight feathers: Warbler.