Antipredator benefits of heterospecific colonial breeding for a predominantly solitary bird

Predation risk profoundly shapes how animals behave and is one of the main forces driving the formation or maintenance of groups. For some species, group living may be facultative, and individuals may live solitarily or aggregate with conspecifics or heterospecifics, but the advantages of each strategy are still poorly known. Here, we investigated whether a predominantly solitary breeding species, the European roller, Coracias garrulus, acquires antipredator benefits from nesting in mixed-species colonies dominated by lesser kestrels, Falco naumanni. We compared the risk-taking behaviour of solitary rollers and rollers breeding in colonies by conducting two experiments. First, we investigated rollers’ latency to resume incubation when presented with a novel object, and second assessed their latency to resume chick provisioning and their investment in mobbing behaviour towards a predator model. We additionally compared the breeding performance and nest predation rate of rollers in each social context (solitary versus colonial) using data from 300 breeding attempts across 6 years. We found that rollers breeding in colonies returned to their nests sooner during the presentation of both the novel object and the predator model and attacked the predator model less frequently than solitary rollers, suggesting they can use heterospecifics as cues in deciding whether it is safe to return to their nests. In addition, rollers in colonies suffered less nest predation than solitary ones, but this did not translate into a higher productivity. Future studies should investigate whether breeding in colonies provides other advantages to rollers, such as increased adult survival or fitness.