A tale of two strategies: Life-history aspects of family strife

Breeding birds can generally be thought of as having evolved life-history t raits that tend to maximize lifetime reproductive success. Within this broa d pattern, many variations are possible because all traits are co-evolved w ith numerous others in complex ways. Clutch-size, for example, has long bee n understood to be frequently lower than the number of young parents are ca pable of supporting by working at their top capacity, especially in long-li ved 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 incenti ves for initial over-production have been identified, with David Lack's res ource-tracking hypothesis having received the most attention. Extra sibs ca n also assist each other in some circumstances, but a third explanation for over-production that has been around for nearly four decades, the insuranc e hypothesis, has been surprisingly overlooked and, in some casts, actively challenged. It simply posits that parents create extra offspring as back-u ps for members of the core brood that chance to die very early. We proposed that the skepticism over thr 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 explana tion for over-production. Some empirical approaches to the study of the ins urance 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.