Role of chemical and visual cues of mammalian predators in nest defense in birds

We explore for the first time the relative importance of chemical and visual cues of predators in nest defense and antipredator behavior in a hole-nesting songbird, the blue tit Cyanistes caeruleus. Birds breeding in nest boxes were exposed to chemical or visual cues of a predatory and a non-predatory mammal during the nestling stage, and their behavior both outside and inside nest boxes was recorded with video films. Our results show that birds respond equally to chemical and to visual cues of predators in a context of nest defense. Adult birds minimized predation risk by decreasing the time spent inside the nest box while feeding nestlings when they were exposed to either visual or chemical cues of a mammalian predator. They decreased the time spent in non-essential activities for nestlings' survival, such as nest sanitation activities, but they maintained provisioning rates so that the nestlings' growth was not compromised. In this way, birds minimized the risk of predation while provisioning nestlings when a predator was detected in the vicinity of their nest.