Interference competition and the functional response of oystercatchers searching for cockles by touch

We examined interference competition for food in oystercatchers, Haematopus ostralegus L., feeding on cockles, Cerastoderma edule L., when kleptoparasitism was infrequent. These birds opened cockles by hammering a hole in the shell, searched for them by touch and experienced densities of feeding conspecifics that ranged from 0 to 2362.5 birds/ha. Handling times were not significantly correlated with competitor density, but the probability of successfully opening a cockle declined significantly as competitor density increased because birds were more likely to abandon cockles they had found. Birds were also significantly more likely to carry cockles away from where they were found prior to attempting to open them as competitor density increased. We used an optimal diet model to predict maximum energy intake rates achievable for birds feeding on a given prey population, and experiencing a range of competitor densities. Despite affecting foraging behaviour, the model showed that competitor density had a negligible impact on overall intake rates. Although kleptoparasitism was rare in our study population, only 1.5% (9/586) of cockles being lost to parasites, a recent model suggests that it was likely to be profitable, under the conditions experienced by our birds. We suggest that kleptoparasitism might be infrequent because birds could reduce its likelihood by adjusting their behaviour, with only a minimal cost in terms of a reduced intake rate. Behaviour-based models of interference competition, therefore, need to consider a range of potentially complex avoidance behaviours when attempting to describe the dynamics of this process. (C) 1998 The Association for the Study of Animal Behaviour.