Once a baby bird hatches, it undergoes several distinct stages before reaching full maturity. Understanding the bird life cycle can aid in identifying birds at different points in their development.
The life cycle of a bird begins with the egg. Female birds lay eggs in clutches that vary in size depending on the species, from just one egg for condors and penguins to as many as 17 for grey partridges. The eggs are incubated by one or both parents until the embryo develops into a chick ready to hatch.
Hatching is no small feat. A chick may take hours or even days to break through the eggshell. Most birds are equipped with an egg tooth, a small, temporary bump on the tip of their beak, which helps them crack the shell and emerge into the world.
A hatchling is a bird that has just emerged from its egg. At this stage, it is often vulnerable, with little to no feathers apart from some fine down. Its eyes may still be closed, and it cannot care for itself.
The term “hatchling” typically applies to altricial birds, which are born underdeveloped and entirely dependent on their parents for warmth and food. These birds, like robins or owls, remain in the nest for some time. By contrast, precocial birds, such as ducks or chickens, are much more developed at birth. They can often feed themselves and leave the nest shortly after hatching.
A nestling is a young bird that is a few days old. At this stage, it is covered in soft down, and you may start to see the beginnings of flight feathers. Nestlings often have their eyes open, but they remain in the nest and rely on their parents for food, protection, and warmth.
Nestlings are unable to leave the nest or fend for themselves and depend entirely on their parents for survival.
As a bird grows and develops its flight feathers and wing muscles, it becomes a fledgling. Fledglings start exploring the world outside their nest but still depend on their parents for guidance and occasional feeding.
At this stage, fledglings may appear awkward and clumsy, hopping about and making short flights. The time it takes for a chick to fledge depends on the species. For example, great frigatebird chicks take six months to fledge and are fed by their parents for an additional 14 months.
Juvenile birds are the awkward “teenagers” of the avian world. They have left the nest and are fully independent, but they have not yet reached sexual maturity.
Juveniles usually sport their first set of feathers, known as juvenile plumage, which is often duller or less distinct than that of adult birds. This can make juveniles difficult to identify in the field. However, the juvenile stage is often brief in many bird species.
Subadult birds are older than juveniles but have not yet developed full adult plumage or reached sexual maturity.
The timeline for reaching sexual maturity varies significantly between species. Small birds like house sparrows may mature in just a few months, whereas larger birds such as golden eagles might take four to five years to reach maturity. Many ornithologists use the terms “immature” and “subadult” interchangeably, although “immature” can refer to any bird that isn’t an adult.
Adult birds are fully mature and capable of reproduction. They exhibit full adult plumage, which may vary depending on the season. For example, some birds develop brighter colours or more defined markings during the breeding season. Others, like puffins, undergo physical changes in the shape or colour of their bills.
At this stage, adult birds are ready to breed and produce eggs, thus beginning the life cycle all over again.
