Field tests of the accuracy of estimating prey size from bill length in oystercatchers, Haematopus ostralegus, eating mussels, Mytilus edulis

Abstract

Field tests were made of the accuracy with which different observers estimated the size of mussels, Mytilus edulis, being eaten by oystercatchers, Haematopus ostralegus. Mussel size was estimated by eye by comparing their lengths with that of the bird's bill. Tests of bias were made in two ways. A ‘natural’ test involved comparing the estimated sizes of prey taken by free-living birds with the lengths of the emptied shells they left on the mussel bed. A ‘model’ test involved estimating mussel size against the bill of a model oystercatcher. Though one observer showed no bias, the other three overestimated the lengths of large mussels and underestimated those of small ones. The magnitude of the bias differed significantly between observers and, to a lesser degree, between the natural and model tests. Because of the curvilinear relationship between prey length and energy content, a bias of the magnitude found in this study could, in extreme cases, lead to serious errors in estimating a prey's energy content. It is concluded that, in studies where prey size is measured against bill length, an observer's individual bias should always be measured, preferably by the natural method. If this is not possible, then tests of the robustness of the conclusions should be made by recalculating the data assuming worst case errors.