Family ties in a neotropical cooperative breeder: within-group relatedness and fine-scale genetic structure in the greyish Baywing (Agelaioides badius)

Abstract

Cooperative breeding is a reproductive system in which one or more adults (helpers) assist others in rearing their offspring. Cooperative breeding occurs in 9% of birds, encompassing remarkable variation in mating systems and patterns of social organization. Understanding how this diversity evolved requires assessing social and genetic relationships across a broad range of cooperative systems. Yet, for some geographical regions like the Neotropics, detailed genetic studies of cooperatively breeding birds are still comparatively scarce. We used double-digest restriction-site associated DNA sequencing to define the genetic relationships within and between cooperative groups in a Neotropical facultative cooperative breeder, the Greyish Baywing Agelaioides badius. Between 2015 and 2018, we banded and sampled adults and nestlings of 41 breeding groups from a population in the Province of Buenos Aires, Argentina. We found that helpers were primarily males, the majority of which were genetically related to the individuals they assisted, either as previous offspring of the breeding pair or as first- or second-order relatives of the breeding male. However, we also observed unrelated male and female helpers. Parentage analysis indicates that reproduction was strongly skewed towards the dominant pair and helpers never gained paternity within and only once outside their social group. This scenario is consistent with helping behaviour being driven primarily by kin selection rather than by direct genetic benefits to helpers. However, the occasional occurrence of helpers unrelated to the breeding pair suggests that non-reproductive direct benefits such as increased access to resources or future breeding opportunities may also influence helping decisions in this species. We found weak evidence consistent with male natal philopatry, and fine-scale genetic structure, as adult males in our population showed higher relatedness at close geographical distances than females. Future studies that further examine the mechanisms behind group formation and the fitness pay-offs of helping behaviour will increase our understanding of the complex cooperative system of the Greyish Baywing.