A Tale of two Strategies: Life-History Aspects of Family Strife

Abstract

AbstractBreeding birds can generally be thought of as having evolved life-history traits that tend to maximize lifetime reproductive success. Within this broad pattern, many variations are possible because all traits are co-evolved with numerous others in complex ways. Clutch-size, for example, has long been understood to be frequently lower than the number of young parents are capable of supporting by working at their top capacity, especially in long-lived species. Nevertheless, studies of species with fatal competition among nestmates have shown that parents routinely create one offspring more than they normally will raise, as if counting on brood-reduction to trim family size after hatching. Three general and mutually compatible parental incentives for initial over-production have been identified, with David Lack's resource-tracking hypothesis having received the most attention. Extra sibs can also assist each other in some circumstances, but a third explanation for over-production that has been around for nearly four decades, the insurance hypothesis, has been surprisingly overlooked and, in some cases, actively challenged. It simply posits that parents create extra offspring as back-ups for members of the core brood that chance to die very early. We propose that the skepticism over the role of insurance is misdirected, that having a back-up is virtually automatic as a contributing incentive to parents and, in some taxa, that it provides a necessary and totally sufficient explanation for over-production. Some empirical approaches to the study of the insurance hypothesis are reviewed, in hopes of encouraging further field study of over-production in general, because that process underlies much of the internal conflict observed in avian families.