In general, fertilized eggs have to be kept in nests where proper care can be taken. The nesting period and the type of nests vary according to the type of species. Nests are built in places, which are inaccessible and are concealed from view. Nests are built in habitats where birds live. Those that live in trees constructed nests among the leaves of trees.
Breeding and care of the young involve a lot of care, tricks and imagination. One of the aspects of bird life is that they have a high rate of metabolism. Raising chicks also involves intense activity on the part of parents. You can easily observe this in the house sparrow and rock pigeons. These two birds very often nest inside or on ledges of houses. When the chicks hatch out, parents have to constantly fly out and bring in food for them. Some birds raise their chicks in specially protected tubes so that they grow rather slowly. Therefore, their food requirement is less. These parents do not have to constantly bring in food for them.
The weaver-bird is an interesting example of such a bird. Among weaver-birds one male mates with several females. The male take up the task of building the elaborate nest. Females take up the responsibility of bringing in food for growing chicks. Weaver-birds construct their nests on a babul or palm tree near a stream or water tank. The male emits a whistling sound to attract the females. When the females are drawn towards the male, the latter performs activities so as to please the female. In this way females are enticed to come to the nest for breeding.
There are many kinds of bird nests. Ducks and lapwings construct their nests on the ground. Manas, parakeets, and woodpeckers find natural holes or cavities in trees and other places to lay their eggs. Swifts and swallows construct their nests on sides of rock cliffs. Their nests are groups of holes in the mud. Weaver-birds use great skill in the construction of nests. Their nests are elaborate suspended structures made up of fibres. Tailorbird is an expert in stitching leaves together to make beautiful nests. It all goes to prove that birds use skill and workmanship to construct their nests.
There is an interesting phenomenon called nest parasitism in which one species of bird lays eggs in the nest of another species. The former does not have its own nest, and therefore, the incubation of the egg is done by the fosters of host in whose nest the eggs have been laid. Koels lay their eggs in the nest of crows. Not all relatives of koel are nest parasites. Coucal is a relative of koel but it raises its own chicks. Coucal is black, but its wings are brownish red and tail is long.
The number of eggs laid in a brood or group varies from 1 to 20. The average period of incubation in small birds is about 12-14 days. The eggs of ostrich take about 45 days to hatch. The longest period of incubation (80 days) has been seen in the royal albatross, which is a sea-bird.
The young or fledgling which hatches may start running about soon afterwards. Such fledglings are called precocial birds. The examples include chickens. But in some cases, the hatched ones have to remain in the nest for sometime before they are able to take care of themselves e.g., house sparrow. These are known as altricial birds. Precocial birds may also be called nidifugous, as they leave the nest soon after hatching. Similarly, altricial birds may be called nidicolous as they remain in the nest for a while.