Oystercatchers and cockles : a predatory-prey study

Abstract

Oystercatchers Haematopus ostralegus and cockles Cerastoderma edule were studied with the aim of understanding the behaviour of predators and the consequences which this behaviour has on the cockle population. At Traeth Melynog, Anglesey, the cockle density declined down the shore and those at the bottom were larger, older and heavier than those at the top. This pattern was caused by the spat settling at the top of the shore combined with downshore movement of some cockles. As a consequence, both flesh content and size were correlated with prey density. The profitability (flesh eaten per minute handling time) of cockles increased with their size. According to optimal foraging theory, these larger ones should therefore have been preferred, and this proved to be so. The rate at which oystercatchers ate cockles followed a type II functional response. The plateau was not caused solely by handling time, nor were satiation or interference important. Analysis of the functional responses of other birds feeding on one prey species showed that in each case the behaviour also conformed with a type II distribution yet neither handling time nor satiation appeared responsible. To account for this, a theoretical model was developed which generates type II functional responses from optimal foraging theory. Another model was developed which predicts the aggregative numerical response for a given level of interference. Because of correlations between density, size and flesh content within the cockle population at I Traeth Melynog, this model could not be directly applied to the data. But the basic assumption of the model - that oystercatchers obeyed the ideal free distribution - could be tested. It gave a poor fit to the data. There was little measurable interference between oystercatchers feeding on cockles. The number of oystercatchers on the Ribble fluctuated in parallel with the cockle population. Much of this change appeared to be due to an influx of young birds. This suggests that young birds seek a suitable estuary whilst adults tend to return to the one found whilst young. Thus the change in oystercatcher numbers was an aggregative numerical response rather than a population numerical response. Due to correlations within the cockle population at Traeth Melynog, profitability reached a maximum value at 25-100 cockles per m2. Therefore the oystercatchers concentrated their feeding at these relatively low cockle densities. As a result, cockle mortality due to predation by oystercatchers was inversely density dependent over most of the range of densities. However, since cockle movement took place, this pattern of mortality could not be detected in the cockle population.