 Many birds molt a few flight feathers at a time, wait until new ones grow in, molt another few, and so on. Usually the same feathers on each wing at the same time, symmetrically. So in summer, a raven that's molting might look a little ragged in the same part of each wing. For ravens, a molt can take up to six months. For smaller birds, closer to two. But, crucially, they can still fly.