Correct answer: Eggs with a calcareous shell
Nearly a century ago T H Huxley called birds "glorified reptiles" thereby meaning that birds have evolved from some reptilian ancestor. Both birds and reptiles lay the same type of eggs, which are deposited outside water. Eggs are large and telolecithal. The ovum is surrounded by albumen,an egg membrane and a thick hard calcareous shell, which are all secreted by special glands located in the walls of oviduct.
Birds like mammals have completely four chambered heart with double circulation,in which there is no mixing of pure and impure bloods. Whereas the ventricle is imperfectly divided in reptiles,resulting in partial mixing of bloods.
All birds have horny epidermal scales confined to the lower parts of their legs and feet,which are exactly like the epidermal scales of the reptiles. Besides birds are covered by feathers,which are homologous to the reptilian horny scales as they have a similar origin and develop from similar germ buds.