ACCURACY OF ESTIMATING THE SPECIES AND SIZES OF OSPREY PREY - A TEST OF METHODS

The accuracies of examining uneaten prey remains collected at feeding sites and of directly observing fish captured while birds forage, comm on methods of determining the species composition and size structure o f prey, in the diets of ospreys (Pandion haliaetus), were tested durin g the summer of 1992 at two shallow lakes in northeastern Scotland. Pr ey remains were collected below feeding perches and the number of head s and paired jaws was used to estimate the minimal number of each spec ies in the diet. Key cranial bones were used for species identificatio n and length estimation. Direct field observations were also made to i dentify the species and sizes of fish taken by foraging ospreys. Fish species were identified by body shape and lengths were estimated by co mparison with the size of the ospreys. The accuracy of field observati ons was tested experimentally using a life-sized model osprey and a se lection of northern pike (Esox lucius) and perch (Perca fluviatilis) o f various sizes. Results showed that the analysis of prey remains gave an accurate estimation of the size range of osprey pre): although sma ll fish (<25 cm) were underrepresented. Tests of field observations sh owed that most fish could be correctly identified on the basis of thei r body shape but there were consistent inter-observer differences in f ish length estimations. These differences should be considered in stud ies using field estimates of prey size, particularly those involving e nergetic calculations where small errors in length estimations can lea d to large errors in estimations of mass and, hence, energy.