Fluctuating environments favour cooperation among non-kin in birds

Cooperative groups are highly variable in relatedness and size, but whether this influences the environments where species live remains unclear. We test the prediction that cooperation among nonkin occurs in extreme environments where the mutual benefits of helping are high. This contrasts to family groups where high relatedness reduces the direct benefits required for helping to be favoured, increasing the environments where cooperation persists. Using phylogenetic analyses of birds, we found that the frequency of cooperation (% nests with 3+ adults) and group size across nonfamily breeders (nspecies=39) increased with fluctuations in precipitation across years. In contrast, cooperative breeding in families (nspecies=128) increased in stable, hot environments and group size did not change with climate. Nonfamily and family cooperative breeders inhabited more extreme environments than phylogenetically matched pair breeders. This shows that cooperative breeding is associated with ecological shifts and that fluctuating climates favour cooperation among nonkin, a pattern not seen in family groups.