Explanation:
The large size of a egg makes it difficult for the female to retain more than a single one egg at a time - carrying eggs would make flying harder and require more energy. (Bird eggs vary in size from the tiny 0.2 gramme eggs of hummingbirds to the enormous 9 kilogram eggs of the extinct elephant bird.)
Just as an aircraft cannot fly if it is overweight, all female birds must dispense with the fertile egg as soon as it is formed. And because the egg is such a protein-rich high-nuitrition prize to all sorts of predators, birds must find a secure place to hatch their eggs. Although birds' eggs appear to be fragile, they are in fact extremely robust. The oval shape applies the same rules of engineering as an arched bridge; the convex surface can withstand considerable pressure without breaking. This is essential if the egg is not to crack under the weight of the sitting bird. It takes 26 pounds of pressure to break a swan's egg and 120 pounds to smash the egg of an ostrich.
