Cooperative breeding (CB) has recently become hot research topic in the field of evolution of animal behavior. However, while CB-related factors have been identified at the species and individual level separately, few studies have used both large- and fine-scale effects to demonstrate CB-related tradeoffs. Here, we propose a “resource gain-fitness loss” model that addresses this question and applied it to the Tibetan azure-winged magpie during their colonization of a high-altitude (i.e., harsh) habitat. We found that CB occurrence decreased with time after colonization and during the breeding season. The decrease coincided with increasing nest security as the threat of predation decreased. Along the same timeline, the relatedness of helpers to the opposite-sexed breeder shifted from kin to non-kin. Cooperative breeders allocated parentage among their social mate, helpers, and outgroup extra-pair mates, with extra-pair mates obtaining larger reproductive shares over time during colonization, indicating that CB facilitated genetic polygamy in the Tibetan population. Male breeders provisioned less with increased female reproductive skew, whereas female breeders’ provisioning rates did not change with male reproductive skew. It appeared that CB occurrence at the population and individual levels were both driven by the same selective force, nest predation, which decreased when other strategies of ensuring nest security were manifested, such as cooperative neighborhoods due to extra-pair copulations. Moreover, our findings suggested the reproductive skew strategy is not a fixed characteristic of the population, but a condition-dependent characteristic of individuals.

We collected the life history data of the azure-winged magpie the field on the Tibetan Plateau. The data about parentage analysis were collected in the labotory. All the data have been processed according to their characteristics.