Have you recently spotted a bird with patchy feathers that looks like it might have lost a feeder battle? It likely is molting - the natural process by which birds replace older, worn feathers with new ones.
Growing new feathers requires a lot of energy, but these costs are outweighed by the benefits of having more aerodynamic or insulating feathers. For birds that migrate or need to withstand coming cold weather, molting is essential. Some species also take advantage of molting to swap between breeding and non-breeding plumage (making their visual identification more challenging and ear birding more valuable - thank you, Haikubox!).
How quickly and when birds molt is highly variable. One study of warblers showed that species molting later did so at a slower pace than those molting earlier, possibly because there is less food available in late-summer to fuel feather growth. Other researchers studied genetic variation between Painted Bunting populations that either molt before or after migration.