Foraging and Size Selection of Mussels by the Tufted Duck, Aythya fuligula

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Abstract
The foraging activities of wintering tufted ducks (A. fuligula) feeding on freshwater mussels (Dreissena polymorpha) were studied to determine whether or not prey-size selection occurred. Eight experiments with captive birds were performed to calculate the profitabiity of the different sizes of the mussels and to ascertain the most preferred size classes by the ducks under controlled conditons. In 2 field experiments the influence of prey density and diving depth upon prey-size selection by wild birds was investigated. The experiments in captivity showed that the ducks did not prefer the most profitable prey, but took significantly smaller mussels. Excluding visual clues in size selection resulted in a further decrease of preferred mussel sizes. Being attached to a solid substrate did not affect the relative profitability of the different size classes of mussels. Selection became more obvious the more size classes offered from each other. The field experiments revealed that both increasing prey density and decreasing depth caused an increase in selectivity. Increasing prey density caused a transition towards preference for smaller mussels. The exact effect of decreasing depth on selectivity could not be identified, probably because the prey density used in the experiment was too low. Diving times of the ducks were strongly affected by both prey density and diving depth. Three hypotheses were proposed to explain the selection of sub-optimal musssels by tufted ducks. The constraints of diving could change the problem of size selection, as the ducks might do better in swallowing 2 smaller mussles in 1 dive instead of 1 larger mussel. The ducks might prefer smaller mussels to avoid the risk of taking mussels that are too large to be swallowed. As larger mussels have highly variable profitabilities, it might pay the ducks to select smaller mussels associated with less variance. The data do not allow the hypotheses to be ranked according to their plausibility.

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