Temporal variation in fitness payoffs promotes cooperative breeding in long-tailed tits Aegithalos caudatus.

Cooperative breeding is paradoxical because some individuals forego independent reproduction and instead help others to reproduce. The ecological constraints model states that such behavior arises because of constraints on independent reproduction. Spatial variation in constraints has been shown to co-vary with the incidence of cooperative breeding in correlational and experimental studies. Here, we examine whether temporally variable ecological constraints can act in a similar way to promote cooperative breeding in the atypical system of long-tailed tits Aegithalos caudatus. In this species, individuals may switch reproductive tactics from breeding to helping within the same breeding season. Using 7 yr of field data, we show that reproductive success declined seasonally because of declines in brood size, nestling weight, and juvenile survival. The survival to breeding age of chicks from nests with helpers was higher than for chicks from nests without helpers, and since helpers usually helped at the nest of a close relative, they accrued inclusive fitness benefits. We used these data to model the expected fitness payoffs of breeding and helping at different times during the season. The model shows that late in the breeding season, the fitness payoff from a kin-directed helping tactic becomes greater than that from independent breeding. The behavioral switch predicted by the model is consistent with the observed switch from breeding to helping, which shows that cooperative breeding may evolve as a way of making the best of a bad job at the end of a temporally constrained breeding season.