Average Clutch Size
size is the number of eggs laid in a single nesting. When
clutch sizes within populations are censused and then the
number of young successfully reared is determined, it often
turns out that the average clutch size is slightly below
that which produces the greatest number of successfully
reared young. One would expect clutch sizes to be
evolutionarily determined to maximize reproduction. Why,
then, would females lay "too few" eggs in a clutch? The
reason is that evolution should, if possible, maximize
reproduction over the lifetime of a female, not reproduction
per brood. By being slightly conservative in the size of the
clutches they produce, females may reduce the stresses of
brood rearing and increase their chances of living through
the following winter and producing more clutches.
Some birds, rather than laying fewer eggs than they can successfully rear, always lay more. Whooping Cranes lay clutches of two eggs, but almost always rear but one young. Presumably, it takes relatively little energy to lay the second egg, even though only one chick can be reared. The second egg is "insurance" against loss of an egg to accident or predation.
Conservationists sometimes take advantage of the insurance egg, removing it from the nest to hatch the chick for captive or foster breeding programs.
in Clutch Sizes;
Asynchrony and Brood Reduction.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.