EFFECTS OF PREDATION BY BIRDS ON GOOSENECK BARNACLE POLLICIPES-POLYMERUS SOWERBY DISTRIBUTION AND ABUNDANCE

The California mussel Mytilus californianus Conrad and gooseneck barnacle Pollicipes polymerus Sowerby are common, competitively dominant organisms inhabiting wave-swept rocky shores along the west coast of North America. Throughout much of their ranges, California mussels are far more abundant than are gooseneck barnacles and previous investigators have cited morphologic constraints and interactions between these species to explain this disparity. Surfbirds Aphriza virgata (Gmelin) and gulls, primarily western gull Larus occidentalis Audubon and glaucous-winged gull Larus glaucescens Naumann, occur each winter in flocks of greater-than-or-equal-to 40-100 individuals along the coast of California. On their wintering grounds, they feed almost exclusively in the intertidal zone and consume large numbers of gooseneck barnacles. A bird-exclusion experiment showed that winter feeding by shorebirds and gulls significantly reduced the abundance of Pollicipes barnacles in plots accessible to birds compared to plots from which birds had been excluded. However, predation by birds was found to be both spatially and temporally patchy. Predation by birds may account for the distribution and abundance of gooseneck barnacles along the coast of northern California. No evidence was found that either physical factors or losses through interspecific competition with the California mussel restricts the distribution of this species.