INSURANCE OFFSPRING AND THE EVOLUTION OF AVIAN CLUTCH SIZE

Surplus offspring can serve as insurance against the accidental loss (congenital defect, infertility, predation, parasitism) of offspring prior to independance. When incipient offspring are cheap to produce, the number of insurance offspring should approximate the number expected to fail. When insurance offspring are not cost-free, their optimal frequency will be determined by the rate of offspring failure, X, and the value of the current offspring relative to the costs of (i) foregone future reproduction (borne by parents), (ii) elimination of redundant offspring (borne primarily by contemporary sibs), and (iii) excessive brood sizes when no mechanism for eliminating redundant offspring exists (borne by both parents and offspring). Insurance offspring seem most likely to occur when clutches are large (increasing the likelihood that at least one offspring fails), where X is high, and where a mechanism for removing surplus offspring exists. Selection for reduced food competition among siblings should favor early elimination of redundant insurance offspring. However, when redundancy of the insurance offspring is ambiguous (e.g. when the surplus offspring serves more than just an insurance function), clemency should be favored. © 1991 Academic Press Limited.