"A Bird is just an egg's way of making another egg"

The Evolution of Eggs

Fish, amphibians, reptiles, birds and even some mammals lay eggs. Eggs are thought to have evolved from the need to protect the embryo from the effects of a harsh, dry environment. Reptiles and birds produce a cleidoic egg, virtually a sealed container. All of the nutrients, water and minerals the embryo needs for development are enclosed within the egg. The only exchange that occurs between the inside and outside of the egg is gaseous. The cleidoic egg may be laid on land, it does not require water as do the eggs of fish and amphibians.

Over time, eggs have become both larger and more complex. A great amount of energy is required to produce large, complex eggs. How do you think the greater investment in an individual egg affects the number of eggs that can be produced?

In some fish, amphibians and reptiles, eggs are kept inside the body until the young are ready to hatch. Birds however, lay eggs long before the young are about to hatch. By laying eggs early in the development of the embryo, the female frees her body from the added weight of the eggs. Why would this be advantageous to a bird?

By laying eggs at an early stage of embryo development, birds must provide heat, protection and periodical turning to make sure the embryos survive. Adult birds commonly have brood patches to help transfer their body heat to the eggs. They move and turn the eggs. As well, they build nests which provide warmth and protection from predators.

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