Functional and numerical response of Oystercatchers Haematopus ostralegus on shellfish populations

Abstract

On a mudflat containing five potential prey species, Oystercatchers predominantly took Cockles and Mussels, probably because the densities and the availability of the alternative prey, Baltic ?Tellin, Ragworm and Shore-crab, were low. The intake rate of Oystercatchers feeding on the mussel bed decreased as water levels dropped and the number of Oystercatchers feeding on the mussel bed declined as more of the adjoining mudflats became exposed. The intake rate of Oystercatchers feeding on Cockles showed a clear positive correlation with the size of the Cockles present, while an effect of the density of the Cockles could not be demonstrated. No evidence was found for interference, i.e. a decrease in intake rate as a result of an increase in bird density. Since the size of Cockles increased downshore, the intake rate of Oystercatchers feeding on Cockles increased when water levels dropped and the birds moved downshore. If Oystercatchers followed the predictions of the most simple ideal free distribution model, they should have all moved to feeding on Cockles when sites below 60 cm NAP uncovered. Though the majority of birds behaved according to predictions, a substantial minority continued to feed on the mussel bed during low tide. This is probably due to individual differences in feeding specialization and, perhaps, local dominance.