Do Tufted Duck and Pochard select between differently sized mussels in a similar way?

Abstract

A series of experiments with 6 captive Tufted Duck Aythya fuligula and 3 Pochard Aythya ferina was performed to test whether or not both species select between different sizes of mussel Dreissena polymorpha in similar way. Standard profitability calculations indicated that the optimal mussel size was, on the average, a little smaller for Pochard than for Tufted Duck, despite the larger bill of the former. Both species selected sub-optimal mussel sizes, but, in contrast to Tufted Duck, Pochard tended to select the smallest mussels available. This led to a significant difference in mussel sizes selected between both species, which may reduce competition in the field. Experiments in which the ducks could select between only two mussel classes were presented. Pochard always took the smallest class. Tufted Duck, on the other hand, tended to select larger mussels, except in situations where both classes offered were proportionally large. Selectivity increased with the difference between mussel sizes available, but Pochard were able to discriminate between large mussels of less than 1.5 mm difference. It is argued that profitability calculations may lead to biased conclusions in situations where animals are faced with less familiar prey.