CAN FOOD SPECIALIZATION BY INDIVIDUAL OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS BE EXPLAINED BY DIFFERENCES IN PREY SPECIFIC HANDLING EFFICIENCIES

Three individually-tested adult Oystercatchers took different proporti ons of shallow-buried bivalves Mya arenaria and Scrobicularia plana fr om a mixture on offer in an experimental situation. Two birds, taking mainly or exclusively Scrobicularia, selected the species as predicted by a random search model based on encounter rate. The third bird acti vely selected for the less abundant Mya. All birds showed size selecti on within the prey species. Diet composition was correlated with the e fficiency of the birds in handling the prey items. Optimal foraging th eory adequately explained the observed prey species- and size selectio n by the birds. A short-term learning effect was found in two of the O ystercatchers. These birds managed to reduce their handling time durin g the course of the experiments, mainly by cutting the flesh loose fro m the shell faster. Cutting was always the most time-consuming compone nt of prey handling but relatively more when a bird took the less pref erred prey species. By rapidly improving their cutting technique for a specific prey, the birds increased their profitability. This adaptive behaviour contradicts the view of the Oystercatcher as a conservative specialist, which has problems when alternative prey should be taken in times of food shortage, although under natural conditions prey swit ching is hampered by morphological constraints.