Shape of the interference function in a foraging vertebrate

1. We provide evidence that interference between overwintering oystercatchers Haematopus ostralegus feeding on mussels Mytilus edulis is absent or has only a negligible effect on intake rate at low competitor densities, and only reduces intake rate above a threshold bird density. The threshold for interference did not differ significantly between individual birds and showed no association with their local dominance. The threshold value was c. 50 birds ha(-1) for birds that opened prey by hammering and 150 birds ha(-1) for those that stabbed into mussels. 2. The interference-free intake rate at densities below the threshold for interference differed significantly between individual birds but was not associated with local dominance; nor did it vary either during the tidal exposure period or between autumn and winter. 3. At competitor densities above the threshold for interference, birds of lower local dominance were more susceptible to interference than those of higher dominance, the magnitude of the difference being smaller in hammering birds than in stabbers. For birds of a given local dominance, interference was most intense towards the end of winter and, in hammerers, at the beginning of the exposure period. 4. We conclude that theoretical models in which interference operates as soon as competitor density starts to increase provide a poor description of this system. Rather, our data support those models in which interference begins only after density has reached a particular, and constant, level.