Response to nest predation risk in a cooperative breeder, the Greyish Baywing (Agelaioides badius)

Cooperative breeding is a reproductive system in which one or more adults (helpers) assist others in caring for young. Helpers typically participate in nest provisioning and defense, but individual contribution to parental care may vary broadly within and among cooperative groups. We studied group and individual responses to nest predation risk using a model presentation experiment in a banded-population of a facultative cooperative breeder, the Greyish Baywing (Agelaioides badius). We presented taxidermic mounts of a nest predator (Milvago chimango) and control species (Mimus saturninus, Paroaria coronata and Colaptes melanochloros) to 13 Baywing nests at the nestling stage. Models were presented sequentially in random order for three minutes, with a 20-min resting interval between presentations. Presentations were filmed and observed from a hide to record the latency to first approach and individual behaviors towards each model. Baywings responded faster and more aggressively to predator than control models. Helper presence had no effect on the latency to approach predator models. Mobbing frequency was higher in the presence of helpers than for breeding pairs alone, but per capita rate of attacks was similar for breeding pairs and trios, suggesting an additive effect of helpers on nest defense. Breeding females mobbed predator models more frequently than males, but individual contribution to mobbing varied broadly both within and among groups. These results suggest that helpers did not improve predator detection but enhance defensive response. Further research on the genetic relationships within cooperative groups will help to explain individual contribution to nest defense.