Although all birds' eggs are oval, there are significant differences between those laid by various species. The more rounded types, for example, are generally laid by birds that nest in holes, or in deep cups where their rolling will not cause problems. Owls' eggs are rounder than most, and the eggs of swifts and swallows are long and narrow. Birds such as guillemots, which nest on precipitous rock ledges, lay sharply tapered eggs that roll in tight circles, and which have shells that are thickened at the pointed end.
The hard shell of a bird's egg is a defence against predators. An inexperienced predator may be discouraged by the difficulty of picking up an object that is so smooth, hard and round. Turtles' eggs, on the other hand, are soft and have a leathery shell. As they are buried in the sand until they hatch, a hard shell is of no benefit to them.
Insects can lay their eggs almost anywhere, because their eggs have very efficient life-support systems. Most have a surface structured so that it traps a layer of air, enabling the embryo developing within to breathe, no matter where the egg has been laid. Insects can therefore deposit their eggs near a food supply and then abandon them.
This may be the reason why further parental care is so rare in the insect world. Among the least caring of insect parents must be the sepsid fly, which lays its eggs in cowpats. Inside, the atmosphere must be suffocating, but each egg is equipped with a long breathing tube that protrudes through the surface of the dung.