RECONSTRUCTING DIET COMPOSITION ON THE BASIS OF FECES IN A MOLLUSK-EATING WADER, THE KNOT CALIDRIS-CANUTUS

Methods are described to assess the molluscan diet of Knots feeding on intertidal flats in western Europe from their faecal output. The size distributions of two common bivalve prey, Macoma balthica and Cerastoderma edule, can be estimated from the heights of shell hinges retrieved from droppings. The average ingested size of the tiny mudsnail Hydrobia ulvae can be reconstructed from partially broken shells. Diet composition in terms of biomass can be estimated by a two-step procedure. First, the diet composition in terms of ash or shell mass is estimated on the basis of the sieved mass of droppings. The application of the site-, season- and size-specific ratios of biomass/shell mass or biomass/ash mass then provides a breakdown of diet with respect to biomass. An exploratory field study in the Wadden Sea showed that measurable fragments in (rarely encountered) regurgitates overestimate prey size. According to the faecal analysis, there was a seasonal change in diet from bivalves to mudsnails with the approach of winter. In combination with field measurements of defecation rates, the outlined methodology may allow the use of droppings as indicators of intake rates.