Passive interference competition by Glaucous-winged Gulls on Black-legged Kittiwakes: A cost of feeding in flocks

We analyzed data from two independent studies of foraging Black-legged Kitt iwakes (Rissa tridactyla) in Prince William Sound, Alaska. Our purpose was to determine if Glaucous-winged Gulls (Larus glaucescens) hindered prey cap ture by kittiwakes. At tightly aggregated feeding flocks, gulls remained on the water directly over the prey and foraged by making brief hop-plunges o r surface-seizes. Kittiwakes, in contrast, fed by diving from the air into open spots in the flock or around its periphery. Data from both studies ind icated that kittiwakes made fewer feeding attempts in flocks that had great er numbers of gulls. However, kittiwake success rate per feeding attempt di d not change as the number of gulls increased. Kittiwakes were more likely to avoid flocks that had a greater number of Glaucous-winged Gulls. Gulls s uccessfully pirated less than 1% of fish captured by kittiwakes. Our findin gs suggest that passive interference may be costly for smaller birds that f eed in multispecies feeding flocks.