How eggs got their shapes: Adaptations for flight may have driven egg-shape variety in birds
(PHYSorg.com Biology)

The evolution of the amniotic egg-complete with membrane and shell-was key to vertebrates leaving the oceans and colonizing the land and air. Now, 360 million years later, bird eggs come in all shapes and sizes, from the almost perfectly spherical eggs of brown hawk- owls to the tear-drop shape of sandpipers` eggs. The question is, how and why did this diversity in shape evolve?