SIZE SELECTION IN DIVING TUFTED DUCKS AYTHYA-FULIGULA EXPLAINED BY DIFFERENTIAL HANDLING OF SMALL AND LARGE MUSSELS DREISSENA-POLYMORPHA

We studied prey size selection of Tufted Ducks feeding on fresh-water mussels under semi-natural conditions. In experiments with non-diving birds, we found that Tufted Ducks use two techniques to handle mussels. Mussels less than 16 mm in length are strained from a waterflow generated in the bill (suction-feeding), while larger mussels are picked up and handled singly. Suction-feeding on small mussels proved to be more profitable. In the non-diving situation, the ducks preferentially took the smallest mussels on offer. It is suggested that smaller mussels incur lower costs of crushing mussel shells in the gizzard. Ducks diving for mussels preferred suction-feeding on all mussels up to about 16 mm in order to minimize the costs of diving. The selectivity for small mussels increased with diving depth (1-5 m), probably because ducks diving deeper spend more time at the bottom collecting small mussels, before a larger mussel is taken to the surface and ingested afterwards. We conclude that the two handling techniques we described for Tufted Duck open up an extensive feeding niche for benthic feeding diving ducks.