As for birds, the egg forms rather slowly within the female; the egg is both relatively big and hard to expel; it has much closer relations with the mother than with the father that fertilized it during a quick coitus; the female is the one who usually sits on it and then looks after the young; but very frequently the father participates in the nest's construction and the protection and nutrition of the young; there are rare cases for example the passerine where the male sits on the eggs and then raises the young.