Keith Grace, age 10, of Greenbrier, Tennessee, for his question:
Why are birds' eggs different colors?
Bird study is a gentle hobby and full of pleasant surprises. Even a beginner learns that a kingfisher egg is snowy white and a buff colored killdeer egg is handsomely freckled with brown. A real expert can tell the dainty differences between eggs of the song sparrow and the field sparrow.
A killdeer's egg, of course, could never hatch into a kingfisher. The magic ingredient in the egg yolk contains a pattern inherited from countless killdeer ancestors. The pattern will be repeated and the egg will hatch into a killdeer chick. Other ancestral patterns also are inherited by the egg. It will be one and a half inches long and smothered with brown spots on a buffish background. Mr. and Mrs. Killdeer make their nest of pebbles on the ground. Their four eggs are placed nose to nose in the center and the ground around the nest is strewn with an assortment of buff and brownish blotched pebbles.
If you walk by the nest, your eye is used to seeing a floor of mottled buff and brown stones and, chances are, you won't notice the matching killdeer eggs in the nest. Neither will a hungry weasel. But if the eggs were red or vivid green, they would catch your attention. But their shape and colors blend in with the background and make them almost invisible. This is called protective coloration. It is one of nature's tricks for protecting her babies by making them almost invisible.
The eggs of the mallard duck are pale buff tinged with greenish grey. The nest on the ground is in tall grass, usually near water. The scenery is a blend of greens and tans and grey flickering shadows. The nest of 12 big eggs blends into the scenery. Each type of bird has a favorite place for nesting high in the boughs, low on the ground or perhaps hidden deep inside a hollow trunk. And the color of the eggs blends in with these favorite surroundings.
The birds that build nests in dark crannies have fewer problems from prying visitors. Their eggs do not have to match their surroundings because they are hidden by darkness. The flicker nests in a gourd shaped hole in a tree trunk. The purple martin nests in a hollow tree or in a dim crevice in the side of a cliff. The kingfisher's nest is hidden in a burrow in the ground. These birds all lay white eggs with dainty designs of protective coloration.
The birds have been around for many millions of years. But it took ages and ages to learn the trick of protective coloration for their eggs. They learned by a process of trial and error. Chicks inherit patterns from their parents. If an ancient killdeer left scarlet eggs among the pebbles, they would be found and eaten. But the children of killdeer who produced speckled eggs had a chance to grow up and lay speckled eggs of their own. This process is called natural selection. Through countless generations, nature patiently selects the most suitable of her little ones to survive.