every one lays eggs. No bird gives birth to live young. Birds quickly form and lay an egg covered in a protective shell that is then incubated outside the body.
Birds developed much great mobility than a mammal, but at the cost of being unable to carry its growing offspring about in its body. Unlike, say, a dog carrying a litter of puppies.
As far as I know, most – but not all – build their nests after breeding. For most of them, it would be a waste of time and energy to build something they were not going to use. However, for some birds nesting isn't necessarily a matter of common sense. In some species building a nest is part of the courtship process,In many species the male bird's skill at nest building is a sign of his suitability as a mate; he invests huge effort in the task. Males of the European house wren build up to 12 nests to attract females. They will continue to build new nests until a female is happy with the construction.
