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

Abstract

One direct benefit of mating outside the pair bond for female passerine birds is to enlist the aid of neighboring males in communal activities such as nest defense. Female passerines are expected to be more heavily invested in offspring than males. The cooperative neighborhood hypothesis indicates that males will participate in in communal nest defense for the public good as they may have sired offspring in neighboring nests. European starlings (Sturnus vulgaris) have unique reproductive strategies with not only males producing genetic offspring outside the pair bond in other nests (extra-pair paternity), but females as well (quasi-parasitism, intraspecific brood parasitism). Therefore, I predicted that both males and females would respond to a predation threat in neighboring nests. European starlings were exposed to a taxidermy mount of a Red squirrel (Tamiasciurus hudsonicus) (experimental treatment) as well as to a similarly sized/shaped rock (control) on the 11-13th day of the nestling period (day 0 is hatch day). An aggregate score of defensive responses (number of; birds, alarm calls and chips, hits and fly-bys to the nest box) was calculated. Significantly extra birds responded to the experimental than control treatment, demonstrating that the taxidermy mount was effective, and that communal defense occurs in this species. Both male and female European starlings participated in mobbing at neighboring nests during the experimental treatment, supporting one of the main predictions of the cooperative neighborhood hypothesis. Therefore, given the mating system of European starlings, it is likely that parental uncertainty in neighboring nests may increase communal nest defense in this species.