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

Cooperative breeding is a common strategy of vertebrates under harsh,
unpredictable environments. Yet we do not fully understand the demographic
mechanisms by which these species buffer 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, without increasing the fecundity variance accordingly.
This pattern held when failed nests were excluded from analysis to filter
the effect of nest predation on the estimation of brood size at fledging,
retaining 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 mitigating
brood reduction by cooperation is a pathway that may explain the observed
pattern in the fecundity mean and variance. Our work highlights the
importance of formulating the mean-variance relationship for fecundity to
understand the evolutionary success of cooperative breeding.