Predicting seasonal and annual fluctuations in the local exploitation of different prey by Oystercatchers <i>Haematopus ostralegus</i>:A ten-year study in the Wadden Sea

We predict the intake rate and prey choice of Oystercatchers feeding along the Frisian coast, Dutch Wadden Sea, combining the optimal prey choice model (Charnov 1976) with detailed measurements of the widely fluctuating food supply. Assuming that the birds maximize their intake rate, the birds should never eat Mussels Mytilus edulis during 10 years of observa- tions, Mya arenaria during two short periods, Macoma blathica and Scrobicularia plana during most summers and Cockles Cerastoderma edule in most winters. Observations on feeding Oystercatchers confirmed the predictions. Due to the seasonal variation in burying depth of Scrobicularia and Macoma, these prey were in winter, if not inaccessible, hardly worthwhile exploiting because of the increase of handing time and searching time with burying depth. Hence, the seasonal variation in intake rate was very large in these deep-living prey compared to surface prey, such as Cockles and Mussels. Consequently, Oystercatchers usually switch from surface to deep-living prey in spring and back to surface prey in autumn in order to maximize their intake rate. Oystercatchers will never achieve a high intake rate when they feed on small prey, even when these prey would occur in extremely high densities. 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 demands juring the limited feeding periods in the tidal habitat. Since Oystercatchers eat only large bivalves, they might be vulnerable because cohorts of prey may disappear completely before they can be harvested. Despite the very large annual variation in the biomass of the different prey species in the Wadden Sea, the total food supply harvestable by Oystercatchers is large enough for them to stay ir, the area, unless ice covers the tidal flats. However, Oystercatchers cannot survive in the Wadden Sea when their diet is restricted to one or two prey species. They need to switch between at least 3 or 4 prey species. For the 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, however. The mortality is higher in winter than in summer and increases with 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 Oystercatchers in the Wadden Sea are limited during circumstances which occur in only some of the winters, viz. when ice covers the feeding areas 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 flesh (AFDW) m(-2), on average. The annual production was 56 g m(-2), but 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 large Mya living out of I-each of the bill, nor the 5 g m(-2) produced by 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 birds. Oystercatchers consumed 12 g m(-2) year(-1), on average, thus more 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 predation pressure by Oystercatchers was much lower iu Scrobicularia and Macoma. In contrast, 80% of the second year Mya was eaten by Oystercatchers in some months. The numbers of Oystercatchers feeding in the study area were weakly related to the annual variations in the total food supply, but strongly related to those of the harvestable food supply. This high correlation must be due to two causal relationships: the bird density increases with the intake rate, and intake rate increases with the harvestable food supply.