Once
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
their eggs. 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. SEE: Incubation:
Heating Eggs;
Temperature
Regulation and Behavior. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.