Attraction of Nesting Wading Birds to Alligators (Alligator mississippiensis). Testing the ‘Nest Protector’ Hypothesis

Abstract

Ecological facilitation (mutualism and commensalism) appears to be a strong force shaping biotic communities, and may be more likely in stressful and dynamic environments like wetlands. We examined a specific type of mutualism, ‘protective nesting associations,’ between herons and egrets (Ardeidae) and American alligators (Alligator mississippiensis). We predicted that wading birds would be attracted to sites with alligators. A survey of potential nesting sites in the Everglades showed strong nonrandom association, with wading birds never nesting without alligators. At previously unoccupied nesting colony sites, we experimentally manipulated apparent densities of alligators and birds using alligator and bird decoys. Small day-herons (little blue herons (Egretta caerulea), tricolored herons (Egretta tricolor), and snowy egrets (Egretta thula)) were significantly more numerous at sites with both alligator and bird decoys than other treatments. These findings together support the hypothesis that wading birds actively choose predator-protected nesting locations based in part on information from both conspecifics and alligators, and suggest that the mechanism supporting this habitat choice is primarily due to nest protection benefits the alligators inadvertently provide. We propose that this interaction is strong and could be geographically widespread, and suggest that it may be critical to shaping management and conservation of wetland function.