Why are ‘birds’ eggs so shaped?

The roundness of an egg allows pressure to be applied from the outside which would break it if applied from the inside. Thus a helpless chick is protected until the moment it needs to break out from its shell. It can then do this with the gentlest of tapping.

Eggs are hatched by an adult bird sitting on them, and the best container for round objects is a cup-shaped nest which prevents them rolling about. So the best shape for eggs is for one end to be smaller than the other. The normal position for eggs in a nest is to have the smaller ends pointing inwards. This means the eggs take up the minimum of room and make it easier for the sitting bird to cover them.

Birds with scanty nests, as in the case or most sea birds, have more elongated eggs. If such eggs are caught by the wind while lying on some cliff-face or rocky ledge, they will spin round instead of rolling over the edge.