a bird's egg is laid, it must be heated if it is to develop.
With rare exceptions, birds use their body (metabolic) heat
to incubate their eggs. This presents them with a problem,
however. Since birds are "warm-blooded," they must be very
careful about losing heat yet be able to transfer heat to
Heat is lost from a bird's surface, and the more surface it has relative to its volume, the more readily it will lose heat. The smaller of two objects with the same shape will always have the greater surface/volume ratio. (A 1-inch cube has a surface area of 6 square inches, and a volume of 1 cubic inch -- a surface/volume ratio of 6/1. A 2-inch cube has a surface area of 24 square inches [6 sides, each with 4 square inches] and a volume of 8 cubic inches. Its ratio then is 3/1 -- only half the surface/volume ratio of the smaller cube.) Most birds are relatively small, and thus have a large surface-to-volume ratio.
One of the main functions of the feathers is to insulate the bird -- to prevent its body heat from being dissipated through the skin surface. Most birds have "solved" the dilemma posed by the need to both transfer and preserve heat by evolving "brood patches." These are areas of skin on the belly that lose their feathers toward the end of the egg-laying period. In most birds the feathers are shed automatically, but geese and ducks pluck their brood patch and use the plucked feathers to make an insulating lining for their nests. The brood patch also develops a supplemental set of vessels that bring hot blood close to the surface of the skin. When birds return to the nest to resume incubating, they go through characteristic settling movements in order to bring the brood patch into contact with the eggs. In precocial birds, after the chicks have hatched the insulating feathers grow back. In passerines, and presumably other altricial birds, the regrowth of the feathers is delayed, and the patches remain functional through early brooding. Then they gradually disappear, restoring the adult's thermoregulatory integrity about the time the young are fledged.
The placement of brood patches differs among groups of birds. There may be a single brood patch in the middle of the belly, as in hawks, pigeons, and most songbirds. Shorebirds, auks, and skuas have one on each side, and gulls and game birds combine these two patterns by having three brood patches. Pelicans, boobies, and gannets have none at all. They cradle the eggs in their webbed feet, cover them with the abdomen, and apparently warm them from both above and below.
When just one parent incubates, it alone develops a brood patch. If both parents incubate, both may grow brood patches, or one may cover the eggs without a patch, warming it less efficiently, but at least retarding heat and water loss from the egg.
Incubation: Heating Eggs;
Temperature Regulation and Behavior.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.