Why are Eggs Oval?

By B Sumangal; Illustration by Sudheer Nath

Have you ever climbed a tree and peeked into the nest of a crow or a sparrow? Or looked into that flowerpot where the noisy pigeon decided to lay its eggs? The sight of a mother hen sitting on a bunch of fresh white eggs is great, though most of us see them only when they land on the breakfast table every now and then.

Eggs come in different colours. They may be blue, blue-green, yellow, spotted, blotched or white. No egg looks identical. Even those eggs that are laid in a clutch or at one time may have different colours. Most eggs are oval, and sometimes they are long and elongated. One end is slightly larger and heavier while the other end is smaller and conical.
The shape of the egg has an important use. It protects the chick inside, until it is time for it to break out of its shell. For, even if you press from the outside, the shell will not break unless the chick inside presses too. The chick does it by tapping from inside with its soft beak when it is ready to face the world.
How does the chick come out? Baby birds in fact have an 'egg tooth'. This is a tiny knob at the tip of the beak to help them break out of the shell. It takes a chick 30 minutes to an hour to break the shell. Some albatross chicks have been known to take five to six days to break out as the shell is so thick!
Most nests are shaped like a large bowl - the right shape for oval eggs to rest without rolling out and falling down. Birds that don't build elaborate nests, like the penguin, the tern and other sea birds, have elongated eggs.
Since these birds lay eggs on rocky ledges or on cliffs, the elongated shapes prevent the eggs from rolling over and breaking on the rocks. Instead they just spin around.
Eggs must be kept warm in order to hatch - about 35 degree centigrade (That is about as warm as an early summer day in northern India). The adult bird provides constant warmth by sitting on the eggs or incubating them. Not only that, the bird keeps turning the eggs around so that all parts of the egg receive the same amount of warmth.
Among most kinds of birds it is the female that incubates the eggs, but in some species it is the adult male which does the hatching. The only bird that doesn't hatch its own chicks is the cuckoo.
The bird is so cunning that it lays its eggs in other smaller bird's nest. Since the eggs look similar the mother bird doesn't realise that it is not hers and sits and hatches the cuckoo's eggs too!
The ostrich lays the largest and heaviest eggs. Each egg weighs around 1.7 kg (3.7 lb) and will not break even if a man weighing 120 kg stands on it. That is how hard the egg is. In fact the eggs are so large that the people living in the Kalahari desert in West Africa use them as a water-bottle!