Cooperative nest defense by European Starlings ( Sturnus vulgaris ) during a predatory threat

Cooperative nest defense has been documented in various passerine species. Parents typically swoop and alarm-call at any predator near their nest, often attracting predominantly male conspecifics to help with nest defense. Potential reasons for males to engage in communal nest defense include direct benefits such as deterring a predator from their own nest area, by-product mutualism (paternity uncertainty in nearby nests), reciprocity, kin selection, and quality advertisement. European Starlings ( Sturnus vulgaris ) are a semicolonial and gregarious cavity-nesting passerine with biparental care. They have a mixed reproductive strategy that includes both extra-pair paternity and intraspecific brood parasitism. Therefore, both paternity and maternity uncertainty could occur in conspecific nests. Our objective was to examine whether conspecific nest defense occurred in this species, and if it did, whether both males and females participated. We exposed adult European Starlings breeding in 16 nest boxes to a taxidermy mount of an American red squirrel ( Tamiasciurus hudsonicus; experimental treatment) and a similarly sized and shaped rock (control) mid-way through the nestling period when nestlings were 11 or 12 days old. Significantly more starlings (parents and conspecifics) responded in the experimental than control trials, and they responded with a significantly higher aggregate score of defensive responses, demonstrating both the effectiveness of the taxidermy mount in eliciting defensive responses and the presence of cooperative nest defense in this species. Both males and females participated in mobbing at conspecific nests during the experimental trials. This study is the first to determine that male and female European Starlings engage in cooperative defense of conspecific nests.