Feathers - Feathers grow back. If your bird accidentally loses some feathers, don't panic. They'll grow back. Bird also molt. It is their way of replacing worn feathers.
Different species molt at different times and lose different amounts of feathers.
Feathers grow back usually in 2-3 weeks. However, if you see bare patches of skin, this may denote a disease or feather plucking. In this case see your vet. Vets - Birds are very different than cats and dogs. Find an avian vet, one who knows about birds.