Prey choice decisions by Glaucous-winged Gulls: Energetic and anti-parasitic hypotheses for size selection of sea stars

Gulls are among few reported predators of the Ochre Sea Star Pisaster ochraceus, an ecologically important intertidal species. However almost no information exists on rates of sea star predation and factors affecting gull foraging decisions. In this study, I report high (up to 90%) occurrences of sea stars in the diets of intertidal foraging Glaucous-winged Gulls Larus glaucescens. Field-based prey choice experiments revealed that the probability of a gull selecting the most energetically profitable sea star sizes was high, but decreased significantly with increasing kleptoparasitism risk. Comparison of optimal diet breadth models suggests that tide related changes in prey availability strongly influence the range of sea star sizes consumed by gulls. Anti-parasitic properties of sea stars, while potentially reducing gull parasite load, appear to have little effect on prey choice decisions. The implications of high rates of size-selective sea star predation by gulls for intertidal community structure are discussed.