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.