Just about all animals grow or develop from eggs.
But not all eggs have shells as bird eggs do. In mammals, a developing baby stays inside the mother until birth. Female birds could not carry chicks inside them and still be light enough to fly. So they—like practically all other animals except mammals—lay their eggs with the baby growing inside. In some species, the newly hatched chicks are born blind and helpless, requiring food that their parents bring to the nest. The chicks of other species can run around as soon as they leave the egg, but they still require the protection and guidance of a parent.
