The foraging skills of Tufted Duck Aythya fuligula and Scaup Aythya marila feeding on Zebra Mussels Dreissena polymorpha were studied in experiments under semi-natural diving conditions with relevance to the IJsselmeer/Markermeer area (large lakes in the centre of The Netherlands, former Zuiderzee area), the major freshwater wintering site of both species in Europe. Daily consumption of mussels (based on fresh mass) was about two to three times the body mass of the birds because of the large water and shell content and consequently low nutritional value of mussels. Feeding costs (diving and processing food) were about 50% of daily energy expenses in winter, as revealed from doubly labelled water measurements. Food intake rates decreased with the degree of byssal thread attachment of mussels in Tufted Duck, while intake rates of Scaup were only affected when mussels grew in tightly attached clumps. Food intake rates were usually 15-25% lower in Tufted Duck than in the 25% heavier Scaup. Both species were able to swallow all mussels available in the population, though smaller mussels were slightly favoured. Scaup had higher food intake rates at low mussel densities than Tufted Duck, but intake rates seemed unaffected at densities higher than ca. 100 gFM m-2, which is similar to giving-up densities observed in the field. Feeding activity consisted of short feeding bouts of a number of dives in quick succession to fill the esophagus with mussels, followed by longer resting pauses of 5-10 minutes to crush mussel shells in the gizzard and digest the flesh. The limited capacity to store food in the gut (less than 5% of the daily requirement) and long digestive pauses imply that diving ducks must spend a large fraction of the day on the feeding grounds. The differences in feeding skills between the two species correspond qualitatively with their spatial segregation in the IJsselmeer area when local differences in prey properties are taken into account.
|Publication status||Published - 1999|