Impacts of predators: Center nests are less successful than edge nests in a large nesting colony of Least Terns

The spatial and temporal patterns of breeding success of Least Terns Sterna antillarum were studied within a large colony at Sandy Point, Connecticut, an ocean beach on the East Coast of North America. Contrary to the 'selfish herd' hypothesis, nests located in the center of the colony suffered from significantly higher levels of predation and had correspondingly lower hatching and fledging success than nests located at the edge (particularly during 1988). Breeding success dropped from 0.56 chicks per nesting attempt during 1987 to 0.08 chicks per nesting attempt during 1988, primarily due to increased predation by Black-crowned Night-Herons (Nycticorax nycticorax). Abandonment levels were proportionally equal for center and edge. Abandonment decreased slightly during 1988 when Black-crowned Night-Heron predation was most intense. Different patterns of predation were observed for the two major predators, Black-crowned Night-Herons and American Crows (Corvus brachyrhnchos). Predation by herons included chicks and eggs, began just prior to peak hatching, and was primarily in the center, whereas predation by crows was confined to eggs and was restricted to the edge of the colony. Results from this study suggest that the different impacts of these two predators may be due to the effectiveness of Least Tern antipredator behavior (viz. mobbing). Least Terns appeared not to mob predatory Black-crowned Night-Herons, but the extent of other antipredator behavior is unknown. The high concentration of Least Terns at the Sandy Point makes this colony particularly vulnerable to predators.