Gull predation is an important source of egg and chick mortality for many seabirds. From 2003-2005, the effects of gull predation and a predator control program on tern nesting success were studied at Eastern Egg Rock, Maine. In 2003, gull predation was uncontrolled, and in 2004 and 2005, attempts were made to shoot Herring (Larus argentatus), Great Black-backed (L. marinus), and Laughing (L. atricilla) gulls that preyed on Common (Sterna hirundo), Arctic (S. paradisaea), and Roseate (S. dougallii) tern adults, eggs, and chicks. To evaluate the effectiveness of gull removal, daily watches were performed from an observation tower and tern hatching and fledging success were measured annually. Despite shooting efforts in 2004-2005, many known predators could not be removed. Great Black-backed Gull predation was a function of year, tidal state, and visibility, while Herring Gull predation depended only on the stage of the tern breeding cycle. Using disappearance of eggs and chicks from monitored nests as a proxy for gull predation pressure, an estimated 23% of Common, 32% of Arctic, and 6% of Roseate tern nests were depredated by gulls during the study period. Predation risk depended on nest position within the colony, but not year, with some areas consistently more vulnerable to gulls than others. We discuss the difficulty of removing predatory gulls from a tern colony lacking nesting Herring and Great Black-backed gulls and suggest the importance of human presence and associated research activities for reducing gull predation at this site.
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Vol. 30 • No. 1