Female birds lay eggs, which serve as incubatory structures for embryos. The incubation period varies by species, typically lasting from 10 to 30 days.

Hatchlings emerge from eggs and are often weak and unable to care for themselves. They rely on their parents for food, which may include regurgitated substances.

Nestlings are a few days old, covered in soft down, and begin to develop flight feathers. They remain in the nest and depend entirely on their parents.

Fledglings start to explore outside the nest and develop their flight muscles. They may appear clumsy and still require parental support for feeding.

Juveniles are independent but not yet capable of breeding. They have their first set of feathers, which are often less vibrant than those of adults.

Adult birds are fully mature and can reproduce. They display full adult plumage, which may change in some species with the seasons.