Quantifying gull predation in a declining Leach’s Storm-petrel ( Hydrobates leucorhous ) colony

. The effect of gull predation on sympatric seabirds has garnered much attention and management action in recent decades. In Witless Bay, Newfoundland, Canada, gulls depredate significant numbers of Leach’s Storm-petrels ( Hydrobates leucorhous ) annually. We quantified this predation on Gull Island in Witless Bay, and its effects on the storm-petrel population, by estimating the annual gull predation rate using strip transects to count storm-petrel carcasses and predicting storm-petrels’ population growth rate by repeating an island-wide breeding census. Using methods that account for island topography, we found that the Leach’s Storm-petrel breeding population on Gull Island declined to roughly 180,000 pairs in 2012 (95% CI: 130,000–230,000), a decrease of 6% per year since the last census in 2001 (352,000 pairs). Based on carcass counts, gulls, mostly American Herring Gulls ( Larus argentatus smithsonianus ), depredated 118,000–143,000 Leach’s Storm-petrels in 2012. Studies of storm-petrel recruitment, the contribution of the large non-breeding component of the population to gulls’ diets, and the consequences of gulls’ storm-petrel diet on the gulls themselves are needed to better predict the trajectory of both species into the future