Most birds are taken care of by at least one parent until they are able to fly and get their own food. The incubation period of bird eggs varies from species to species. There's also some variability due to the temperature. Some birds, like cuckoos and some cowbirds, lay their eggs in other bird's nests. The non-related adult bird takes care of the cuckoo's egg unwittingly.
Some cuckoos even kill the other eggs in the nest to insure that their egg gets enough food.
Many bird species migrate to a cooler climate for reproducing and summer feeding, and then return to a warmer climate for the winter. It is unknown exactly how birds are able to navigate accurately , but many people are studying this mystery of how birds know where to fly. Birds belong to the biological class Aves and live virtually everywhere on Earth. There are about 9,000 different species of birds, divided into 24 orders and 146 Families . Early birds, like Archaeopteryx , had teeth in their mouth and claws on their wings.