Feathers serve several functions in birds: Besides their role in flying, feathers are also important for body insulation and water repelling (which also reduces the chance of losing body heat).
Because feathers wear away, new ones need to be produced regularly. The replacement of feathers is called moulting. Moulting costs birds a lot energy and different bird species employ different strategies for it. Birds usually do not moult during the breeding or migration season. These are already rather exhausting times for the birds. Also winter, with its scarceness of food, is not a good time for moulting. Most bird species moult in late summer, between breeding and migration. During this period, there is also less singing and the birds are less visible. A complete moult costs a lot of energy and reduces flying capacity. The birds hide in dense bushes and trees. A complete moult is seen in, for example, ducks, geese and rails. After moulting, the feathers are in optimal condition for migration. Some birds species moult a second time, just before the breeding season. In birds of prey, for example, the moulting of pin feathers happens gradually, all year through. This allows them to keep flying. Only during migration and the breeding season moulting is halted.