Almost all animals start in eggs.
But not all eggs have shells like bird eggs. In mammals, a developing baby stays inside the mother until birth. Female birds could not carry chicks inside and still be light enough to fly. So they lay their eggs with the baby growing inside. In some species, newly hatched chicks are blind and helpless. The parents have to bring food to the nest. The chicks of other birds can run around as soon as they leave the egg. But they too still need protection and help from a parent.
