Depth of sampling corer for estimating the food supply of wading birds Charadrii: when is it important to dig deeper?

When sampling the intertidal macro-invertebrate food supply of wading birds Charadrii, cores of sediment have been taken to depths of 15–30 cm or even more. But for a variety of reasons, samples are often taken to 15 cm. Using data from the Dutch Wadden Sea on the depth-distribution of one widely important prey species—the ragworm, Hediste diversicolor—we explore the circumstances in which the shallower corer might fail to sample an important part of the birds’ food supply. The risk of excluding substantial amounts of ragworms, and therefore of under-estimating the biomass density food supply, is greatest in winter, in sandier sediments and when large worms are in high densities. Only the estimate of the biomass density of small ragworms consumed by Dunlin Calidris alpina and Common Ringed Plover Charadrius hiaticula would be unaffected at any time of year by sampling with a 15-cm corer. The mean size of the average worm present, which largely determines intake rate in shorebirds, might also be underestimated, although this proved not to be a serious bias in the particular study area considered. We recommend that preliminary studies are always carried out to establish whether, in a particular place at a particular time, a 15-cm corer is sufficient to obtain an accurate measure of both the biomass density and the mean size of the individual ragworms present, or whether a deeper corer should be used.