HOW THE FOOD-SUPPLY HARVESTABLE BY WADERS IN THE WADDEN SEA DEPENDS ON THE VARIATION IN ENERGY DENSITY, BODY-WEIGHT, BIOMASS, BURYING DEPTHAND BEHAVIOR OF TIDAL-FLAT INVERTEBRATES

For several reasons, waders in the Wadden Sea face a large seasonal an d annual variation in their food supply. Observations on a tidal flat in the Dutch Wadden Sea have shown that: (1) The average energy densit y of ten invertebrate prey species varies between 21 and 23 kJ.g-1 AFD W. In Scrobicularia plana and Mya arenaria, but not in Macoma balthica , the energy density is 10% lower in winter than in summer. (2) Depend ing on the species, body weights of prey of similar size are 30 to 60% lower in winter than in summer. (3) The year-to-year fluctuation in s tanding-crop biomass is larger in some species than in others, the dif ference depending mainly on the frequency of successful recruitment. T he overall biomass of the macrobenthos in winter is half of that in su mmer, but the timing of the peak biomass differs per species. (4) The burying depth varies per species: Cerastoderma edule live just beneath the surface, while M. balthica, S. plana, M. arenaria, Arenicola mari na and Nereis diversicolor bury more deeply and the majority of these prey live out of reach of the bird's bill. In all six species, burying depth increases with size. There is no seasonal variation in depth of C. edule and M. arenaria, but the four other species live at most sha llow depth in early summer and most deeply in midwinter. Burying depth s in winter vary from year to year, but are unrelated to temperature. Neither has temperature any effect on depth within months. For knot Ca lidris canutus feeding on M. balthica, the fluctuation in the accessib le fraction was the main source of variation in the biomass of prey th at is actually harvestable, i.e. the biomass of prey of suitable size that is accessible. Accordingly, the paper reviews the available data on the temporal variations in accessibility, detectability, ingestibil ity, digestibility and profitability of prey for waders. Only a small part of the prey is harvestable since many accessible prey are ignored because of their low profitability, while many profitable prey are in accessible. The profitability of prey depends on their size and weight but also on their depth in the mud, since handling time increases wit h burying depth. A simple biomechanical rule explains why the handling time of small prey increases with bill length and why large, long-bil led waders ignore a disproportionately larger part of the small prey. The fraction detectable for visually feeding waders is usually very lo w, especially when the temperature of the substrate is below 3-6-degre es-C. Waders vary their prey choice over the year in response to the c hanges in the availability and profitability of their different prey s pecies. The food supply harvestable by waders is much lower in winter than in summer. For waders wintering in the Wadden Sea, the food suppl y may be characterized as unpredictable and usually meagre. Waders win tering in NW Europe are concentrated in coastal sites where the averag e surface temperature is above 3-degrees-C. This probably cannot be ex plained by a greater burying depth, and only partly by a lower body co ndition, of prey in colder areas. Yet the harvestable fraction is lowe r in colder sites, especially for sight-feeding waders, as invertebrat es are less active at low temperatures. However, the lower energetic c ost of living and reduced chances of the prey being covered by ice may also contribute to the waders' preference for warmer sites.