Why do many birds molt all their feathers at once or in stages?

Feathers are durable but not permanent, and sunlight, friction, and weather slowly damage them. Molting replaces worn feathers so birds can keep flying efficiently and stay insulated. Many birds molt in stages, swapping a few feathers at a time so they can still fly and forage normally. Other species, such as some ducks, may lose flight feathers all at once and become temporarily flightless while they hide in safe habitat. The timing of molt is often tied to breeding season, migration, and food availability, because losing feathers can be energetically expensive and risky.