- Precocial
- Altricial
There are 2 main development patterns in young birds as they hatch :
Precocial
Early development. Youngs are covered with down feathers, their eyes and ears are functional shortly after hatching, and their legs can support them. Some leave the nest immediately, others after 1-2 days, most following their parents for some time. Other can swim and even dive in the first days.
Colors of their down feathers are often similar to those of the ground. Legs are proportionally longer than those of adults.
The young hatch nearly simultaneously, so that they leave the nest with 1 or 2 adults shortly after, and they can already feed themselves, although the parents often help them for some time.
Most nest directly on the ground, hens, partridge, pheasant, waders, ducks, swans, mergansers, etc.
The nest can be a simple hollow on the ground, or decorated with grass, leaves.
Altricial
Meaning "requiring nourishment". They hatch naked, except sometimes for some down feathers on the head, or some feather structure on the wings. They are dependant on the adults to stay warm or for protection from the sun, and for food.
Their first down feathers grow quickly, offering some thermal insulation, and allowing the parents to leave the nest to get some food. Still blind and unable to stand on their legs - which would be dangerous as they could fall from the nest - their activity is limited to opening their beaks and chirp as soon as they feel an adult is coming to the nest.
Once their first feathers have replaced down, they can perch near the nest, still fed by the adults, then explore the neighborhood with their parents who still help them for food.
Those who nest on cliffs or steep rock faces, like some vultures and birds of prey, need to train their flight muscles before takinf off for the first time, flapping their wings until they feel they can support their own weight.
The nest is often build with twigs, the size and complexity differs a lot depending on the species, and can be placed at various heights, near the ground, in bushes, trees, cliffs, etc.
Blackbirds, pigeons, falcons, storks are examples of altricial birds.
Cavernicolous Some nest in hollows in trees, burrows, and as they are more protected, they are usually the less developed when they hatch.
Some species build a nest inside the cavity, but other just lay their eggs on the wood or ground.
As there are sometimes no perch near the nest, some young birds like Bee-eater can fly without any previous exercice for their wings.
Woodpeckers, tits, kingfishers...
There are also intermediate forms of developments, or atypic behaviors, like cukoo's parasitism of other species brood, where the egg is laid in the nest of another bird.
