Although egg predation from aerial predators is one of the most important threats to ground-nesting seabirds, few populations conceal their nests under vegetation. However, little is known about the relationships between nesting habitat characteristics, nest densities and egg predation. Dry Tortugas (DRTO) Sooty Terns (Onychoprion fuscata) nest on sandy substrate. However, a substantial shift in the onset of their breeding season has increased their exposure to aerial predators, which, in turn, may have triggered a change in nesting behavior. A Bayesian model was developed to determine the relationship between vegetation cover and nest numbers, and whether such a relationship has reduced the incidence of egg predation. Over eight years, data on the number of nests, incidents of egg predation and on vegetation cover were collected from 132 survey plots. A clear non-linear relationship between vegetation cover and nesting numbers was found: thresholds of 65% cover of woody vegetation, such as shrubs, and of less than 13% of forbs, were shown to maximize the number of nests, and to significantly reduce egg predation. These results have implications for nesting habitat management and demonstrate that Sooty Terns are capable of changing their breeding behavior in response to an increase in the risk of egg predation.
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Vol. 33 • No. 2