The Bird Egg Season in Alaska
They’re back. After six months away, migratory birds are flapping and fluttering to Alaska’s forests, lakes, and shorelines. Soon, the snowbirds will decorate the northern landscape with millions of eggs.
In The Birder’s Handbook, editors Paul Ehrlich, David Dobkin, and Darryl Wheye ponder the study of bird eggs, known as oology. They point out that every animal with a backbone produces eggs, but reptiles and their descendents, birds, somewhere along the line enclosed their eggs within a shell.
An eggshell is an incubator with a snack bar, providing everything an embryo bird needs except warmth and oxygen. Within the shell floats the fatty yoke, on top of which clings the bird embryo. The embryo feeds on the yoke and sucks water from the albumen, also known as egg white. Oxygen enters the egg the same way carbon dioxide and water get out, through tiny pores in the eggshell. An average chicken egg has about 7,500 of these microscopic holes.
To develop into a bird, an egg needs to stay as warm as its mother, roughly the same as human body temperature. Mother birds provide this heat by plucking themselves bare in a spot, known as a brood patch, that will touch the eggs when mothers sit on the nest. Perhaps to ensure the eggs receive an even warming, mother birds often turn their eggs in the nest.
Some birds emerge from the egg more ready to take on the world than others. Megapodes, birds that live in Australia and on other Pacific islands, hatch with almost everything they need to fend for themselves. Rather than sit on eggs, mother megapodes bury the eggs in rotting vegetation. The heat of decomposition incubates the eggs. When fledgling megapodes peck their way free from their eggshells and dig out of the muck, the birds are feathered, almost ready to fly, and quite alone. The anti-megapode is the house sparrow, which is born naked, blind, and helpless. Researchers have found that birds that fend for themselves shortly after hatching emerge from eggs that contained a high percentage of yolk. Megapode eggs contain about 67 percent yolk, while mothers of needy young birds like the house sparrow invest much less in egg yolk, about 25 percent.
Bird eggs range from the size of shot puts (ostrich eggs) to smaller than aspirin tablets (hummingbird eggs). Not all birds lay the “egg-shaped” eggs of a chicken. Owl eggs can be round as golf balls. Hummingbird and swift eggs tend to be shaped more like a capsule, long and narrow. Shorebirds and birds that nest on cliffs often lay eggs pointed on one end and rounded on the other, a teardrop shape when viewed from above. Some biologists think this shape allows the mother bird to cover eggs more efficiently when the pointed ends face inward, making the eggs fit together snugly. The shape also makes the eggs roll in a circle, which may be an advantage to birds nesting on flat, narrow ledges.
Eggs vary in color from the pale white of tree-cavity nesters to the camouflaged speckles of species that nest on the ground. One of the great mysteries of oology is the American robin’s sky blue egg, which seems an easy target for nest raiders.
Another good question is why produce shelled eggs at all? Why don’t birds give birth to live, noisy babies, as do humans? Some scientists say birds’ bodies are too hot for developing embryos. Maybe live birth isn’t all it’s cracked up to be anyway. Eggs with shells seem to be working fine for the birds.
