Risk Assessment During Nest Defense Against Three Simulated Predators by Female Northern House Wrens (Troglodytes aedon)

Abstract

Offspring predation is one of the greatest obstacles to an organism's reproductive success, but parents vary in the strength of their response to potential predators. One explanation for this variable investment is that defending current offspring has the potential to lower future reproductive success if the predator is also capable of injuring or killing the parent. Northern house wrens (Troglodytes aedon) are cavity-nesting songbirds that defend against multiple species of nest predators including small mammals, birds of prey, and snakes. Here, we used three different predator decoys: two nest predators—an eastern chipmunk (Tamias striatus) and an eastern ratsnake (Pantherophis alleghaniensis)—as well as a predator of both offspring and adults—a juvenile Cooper's hawk (Accipiter cooperi)—to elicit nest defense and test whether females use risk assessment to modulate their antipredator behavior. We found that antipredator behaviors were not significantly different between the two nest predators, which posed a high risk to the nestlings, but lower risk to the parents, as neither species frequently captures adult wrens outside the nest box. However, female wrens never dove at or attacked the Cooper's hawk, while they frequently attacked both the snake and chipmunk decoys. Neighboring house wrens from adjacent territories were also less likely to respond to the hawk, but more heterospecifics mobbed the hawk than the snake decoy. Collectively, these results show that risk assessment and the strength of the antipredator response varies substantially both within and among species. Female house wrens exhibit plasticity in their nest defense behavior, and they respond to different types of predators in a way that could maximize lifetime fitness while risking the loss of their current offspring.