Cooperative breeding in birds increases the within-year fecundity mean without increasing the variance: A potential mechanism to buffer environmental uncertainty

Abstract Cooperative breeding is a common strategy of vertebrates under harsh, unpredictable environments. Yet we do not fully understand the demographic mechanisms by which these animals buffer the environmental challenges, because while cooperation typically increases the fitness mean, whether it alters the variance to further reduce the population risk of stochastic extinction is unclear. We addressed this issue using a meta-analysis among avian cooperative breeders. Compared with their non-cooperative counterparts in the same populations, cooperative pairs on average fledged more offspring, but the fecundity variance did not increase accordingly. This pattern held when failed nests were excluded from analysis to eliminate the effect of complete nest predation on the estimation of brood size at fledging and retain the effect of brood reduction (the partial loss of nestlings in a brood). Cooperative pairs did not differ in the mean and variance of clutch size from non-cooperative pairs; the former had a lower level of brood reduction than the latter. These results suggest that reducing nestling mortality in small-sized broods is a potential pathway through which cooperation increased the fecundity mean but did not cause the variance to increase. Our work highlights the importance of formulating the mean-variance relationship for fecundity to understand the evolutionary success of cooperative breeding.