Nest Attentiveness in Common Terns Threatened by a Model Predator

Abstract

Nest defense theory interprets the behavior of animals faced with the threat of predation within an optimality framework in which risks to parents are balanced against risks to offspring. We conducted “flights” of a model Peregrine (Falco peregrinus) towards nesting Common Terns (Sterna hirundo). Nesting parameters such as laying date, clutch-size (one to four eggs), and offspring age (fresh eggs to newly-hatched chicks) were known for 57 individual parents, while egg mass (16.5-24.6 g) was known for 50 parents. Age (three to 23 yr) and sex were known for 27 and 41 parents, respectively. We defined nest attentiveness as the number of outflights performed by each bird in the first 15 min after the Peregrine flight. Multivariate analysis showed that high attentiveness was associated with early laying and large eggs, but not with sex of the parents, distance from the predator, nor with clutch-size. Age of parents and age of offspring were less strongly correlated with nest attentiveness, but could not be excluded as explanatory variables. Among existing hypotheses, our results are most consistent with the hypothesis that nest attentiveness should increase with increasing value or quality of offspring. Given that previous studies have shown early laying and large eggs to be indicators of high parental quality in terns, we propose a new hypothesis that the intensity of nest attentiveness also increases with parental quality.