PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITATIONOF DIFFERENT PREY BY OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS - A 10-YEARSTUDY IN THE WADDEN SEA

We predict the intake rate and prey choice of Oystercatchers feeding a long the Frisian coast, Dutch Wadden Sea, combining the optimal prey c hoice model (Charnov 1976) with detailed measurements of the widely fl uctuating food supply. Assuming that the birds maximize their intake r ate, the birds should never eat Mussels Mytilus edulis during 10 years of observa- tions, Mya arenaria during two short periods, Macoma blat hica and Scrobicularia plana during most summers and Cockles Cerastode rma edule in most winters. Observations on feeding Oystercatchers conf irmed the predictions. Due to the seasonal variation in burying depth of Scrobicularia and Macoma, these prey were in winter, if not inacces sible, hardly worthwhile exploiting because of the increase of handing time and searching time with burying depth. Hence, the seasonal varia tion in intake rate was very large in these deep-living prey compared to surface prey, such as Cockles and Mussels. Consequently, Oystercatc hers usually switch from surface to deep-living prey in spring and bac k to surface prey in autumn in order to maximize their intake rate. Oy stercatchers will never achieve a high intake rate when they feed on s mall prey, even when these prey would occur in extremely high densitie s. The reason for this is that the yield of small prey during handling is even less than the intake rate during feeding of 1 mg ash-free dry weight (AFDW) s(-1) which Oystercatchers need to meet their energy de mands juring the limited feeding periods in the tidal habitat. Since O ystercatchers eat only large bivalves, they might be vulnerable becaus e cohorts of prey may disappear completely before they can be harveste d. Despite the very large annual variation in the biomass of the diffe rent prey species in the Wadden Sea, the total food supply harvestable by Oystercatchers is large enough for them to stay ir, the area, unle ss ice covers the tidal flats. However, Oystercatchers cannot survive in the Wadden Sea when their diet is restricted to one or two prey spe cies. They need to switch between at least 3 or 4 prey species. For th e same reason, the birds have to roam over feeding areas measuring ar least some ten's of km(2). The winter remains a difficult period, howe ver. The mortality is higher in winter than in summer and increases wi th the severity of the winter. Besides, the winter mortality increases when the food consumption is reduced, due to either a low intake rate and/or a short feeding time. Therefore, the wintering numbers of Oyst ercatchers in the Wadden Sea are limited during circumstances which oc cur in only some of the winters, viz. when ice covers the feeding area s and the harvestable food supplies are low. The total biomass of the five bivalve species in the study area amounted to 81 g ash-free dry f lesh (AFDW) m(-2), on average. The annual production was 56 g m(-2), b ut only 32 g m(-2) can be considered as exploitable by Oyster catchers . Oystercatchers did not han est the 9 g m(-2) year(-1) produced by la rge Mya living out of I-each of the bill, nor the 5 g m(-2) produced b y bivalves too small to be eaten by Oystercatchers. Moreover 9 g m(-2) disappeared during disasters (e.g. frost) and could not be eaten by b irds. Oystercatchers consumed 12 g m(-2) year(-1), on average, thus mo re than the 10 g m(-2) taken by all other shorebird species together. Half of the prey biomass disappearing due to mortality between August and March could be attributed to Oystercatcher predation. The predatio n pressure by Oystercatchers was much lower iu Scrobicularia and Macom a. In contrast, 80% of the second year Mya was eaten by Oystercatchers in some months. The numbers of Oystercatchers feeding in the study ar ea were weakly related to the annual variations in the total food supp ly, but strongly related to those of the harvestable food supply. This high correlation must be due to two causal relationships: the bird de nsity increases with the intake rate, and intake rate increases with t he harvestable food supply.