Defending the Weak: Parental Defense Peaks When Chick Vulnerability is Greatest in the Herring Gull (Larus argentatus)

Abstract

AbstractSuccessful reproduction often depends upon parents providing offspring with resources and protection. In birds, reproductive success can often be enhanced by parents engaging in antipredator behaviors, but these behaviors can be costly. Theoretically, individuals should temporally modify the intensity of nest defense behavior to balance the costs and benefits of current and future reproductive success. More specifically, nest defense should vary throughout a nesting attempt to maximize fitness of the adults. Here, we consider the relationship between nest defense behavior and chick vulnerability in the herring gull (Larus argentatus), where chicks are under high predation risk. We estimated chick vulnerability by quantifying survival probabilities at different periods of the nestling stage. Simultaneously, we quantified changes in parental aggression throughout the nesting cycle by simulating predation attempts using a human predator model. We found that chick survival probabilities were lowest (i.e., vulnerability was highest) and parental aggression in nest defense was greatest during the first 10 days after hatching. Thus, we show that parents are most defensive when chicks are most vulnerable and that adults optimize nest defense behaviors in a way that maximizes their fitness.