* Hard outer shell: The eggs of birds have a hard outer shell made of calcium carbonate, which protects the developing embryo inside.
* No placenta: Bird eggs do not have a placenta, which is an organ that provides nutrients and oxygen to the developing embryo in mammals. Instead, the embryo is nourished by the yolk sac, which is a sac of nutrients that is attached to the embryo.
* Incubation period: Bird eggs are incubated by the parent bird, which provides warmth and protection for the developing embryo. The incubation period varies depending on the species of bird, but it can range from a few weeks to several months.
Baby mammal:
* No hard outer shell: Baby mammals are born without a hard outer shell. Instead, they are born with a soft, pliable body.
* Placenta: Baby mammals have a placenta, which is an organ that provides nutrients and oxygen to the developing embryo. The placenta is attached to the mother's uterus, and it allows the embryo to exchange nutrients and oxygen with the mother's bloodstream.
* Gestation period: Baby mammals are born after a gestation period, which is the time from conception to birth. The gestation period varies depending on the species of mammal, but it can range from a few weeks to several months.