Cooperative partnerships in azure-winged magpies transition from kin-based to non-kin-based during the period of colonization of the Qinghai-Xizang Plateau

Abstract

Cooperative breeding exhibits considerable variation in the conditions in which it occurs and the relationship between dominant breeders and helpers. To explore factors influencing the dynamics of cooperative breeding, we investigated cooperative breeding in azure-winged magpies Cyanopica cyanus during their colonization of a high-altitude environment. Cooperative breeding occurred more frequently in colonies with higher nesting density, while it did not vary with the environmental proxy (i.e. nest habitat tree-ring index). Those nests with delayed clutch initiation or larger-bodied social males were less likely to adopt cooperative breeding. These results suggest that social factors play a more critical role than natural environmental conditions in driving the occurrence of cooperative breeding. Although both sexes might act as helpers, males did so more frequently than females, and females unrelated to social breeders became helpers more often than unrelated males. The likelihood of helpers unrelated to either parent rose significantly over the period of colonization, suggesting that cooperative partnerships transition from kin-based to non-kin-based over the time of azure-winged magpies' colonization of high-altitude habitats. Accordingly, unrelated helpers gained greater shares of reproductive partitioning compared to those related to one or both parents. Given that azure-winged magpies face high risks of predation from natural predators or conspecific nest-raiders, the higher occurrence of nest predation by predators – but not by conspecific nest-raiding – in biparental nests compared to cooperative groups indicates that cooperative breeding helps reduce predation pressures from natural predators, but not from conspecific raiders. Therefore, cooperative breeding facilitates azure-winged magpies' successful colonization of novel high-altitude habitats.