Eggs provide the essential nutrients that the embryo needs for proper growth. Oviparous animals produce eggs that hatch after leaving the mother’s body. Mammals and other groups have evolved the ability to keep embryos within the reproductive tract until development is complete. One of the challenges of this baby-making method called viviparity is that babies are heavy to carry around.
Scientists suggest viviparity has yet to evolve in any species of bird because mother birds can’t fly with weighty developing embryo passengers. A theory suggests that bird embryos get more air and develop into healthier chicks laying in a nest than they would inside the mother's body.