The Cornell Lab of Ornithology identifies several categories of molting behavior. Species including hawks, owls, woodpeckers, hummingbirds, flycatchers and thrushes have one complete molt per year, when they replace all of their feathers. These birds’ appearance is the same year-round. The second category includes birds such as warblers and tanagers that have one complete molt, and one “prenuptial” molt where only body feathers are exchanged. Males in particular don brighter, bolder colors for breeding season. The complete molt takes place after breeding season and before migration. Gambel’s quails fall into this group, although quails only exchange a limited number of body feathers in the early spring prenuptial molt. The final and rarest pattern is for two complete molts per year, for birds that live in areas where their plumage takes a lot of wear and tear. Marsh wrens and bobolinks are among this category of birds that encounter harsh vegetation as they forage.
Young birds are identifiable by their immature feathers. Passerines, or perching birds, hatch naked and grow their first feathers or juvenile plumage in their first few weeks. Non-passerines hatch with a natal down covering their bodies. These downs are replaced by juvenile plumage in what’s termed a postnatal molt. Most passerines come into their adult plumage or definitive plumage around one year of age. Larger birds do not reach plumage maturity for two years or more. Gulls and eagles take the longest, with complex stages of molting and plumage coloration. Gulls generally acquire definitive plumage at four years, and bald eagles at five to six years.
The American kestrel is one of the only birds of prey that exhibits sexual dimorphism, and is unique in that nearly adult-like plumage is attained prior to fledging. Thus the males and females of this species can be identified early on, due to the coloration of their feathers. Kestrels have an incomplete molt in the fall of their first year, but retain feathers in their wings and tail and parts of their bodies until the second and complete molt the following year. By winter of their second year, the young falcons have attained their definitive plumage.