Annual and seasonal variation in the food supply harvestable by knot Calidris canutus staging in the Wadden Sea in late summer

• Published in: Marine Ecology Progress Series
• Main Authors: Zwarts, L., Blomert, Anne-Marie, Wanink, J.H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1992
• Subjects: LONG-TERM CHANGES; TIDAL FLAT AREA; SPECIES RICHNESS; CERASTODERMA-EDULE; MILD WINTERS; BIOMASS; ANIMALS; WADERS; PREDATION; PERIOD; Burrows; Calidris canutus
• Online Access: https://hdl.handle.net/11370/9cc406b6-5e82-4c1a-8c0b-f5eda201e91a; https://research.rug.nl/en/publications/9cc406b6-5e82-4c1a-8c0b-f5eda201e91a; https://doi.org/10.3354/meps083129; https://pure.rug.nl/ws/files/62553650/Annual_and_seasonal_variation_in_the_food_supply_harvestable.pdf

The biomass of the macrobenthic animals living in intertidal flats of the Wadden Sea varies annually and seasonally. However, the variation in prey biomass harvestable by wading birds such as knot Calidris canutus, which feed mainly on the middle range of their prey size classes, is even larger. The majority of first-year Cerastoderma edule, Mya arenaria or Mytilus edulis are too small to be profitable as prey for knot. Yet, by the end of the subsequent growing season, these same prey are too large to be ingested and/or live at a depth that puts them out of reach of the birds' bills. Macoma balthica is a major prey for knot, because (1) its annual spatfall is less erratic than in the other bivalve species, and (2) it grows more slowly, and each cohort is therefore available as food for knot for at least 3 yr. Knot feed in flocks which roam over the feeding area, but they are more often observed in food-rich than in food-poor areas, A similar relationship between bird and food densities was found in one locality, when the annual numbers of knot were compared with the yearly variation in food supply. Since the numbers of knot in the whole area were the same over many years, the birds were apparently able to find other feeding areas when the local food supply was low, i.e. the food supply harvestable by knot (prey not too small, not too large and not too deep) was less than about 4 g ash-free dry weight m-2. Knot arrive in the study area at the end of July and leave after only a few weeks en route to Africa. They depart before d serious decline in the harvestable prey biomass takes place, which results from a decrease in the body condition of individual prey and an increase in the fraction of the prey which burrows out of the reach of the knot's bill.