Average Clutch Size

Clutch 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.

SEE: Variation in Clutch Sizes; Natural Selection; Hatching Asynchrony and Brood Reduction.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.