Thursday, August 12, 2010

Taking Smaller Bites

Male Tufted Duck, Aythya fuligula

An attractive little black and white bird, the Tufted Duck (Aythya fuligula) is to be found all year round on lakes, resevoirs and flooded pits where it can be seen diving for food (Chinery, 1987 Field Guide to the Wildlife of Britain and Europe p. 75). The male (an immature example of which is pictured) has a black back, neck, head and tail with a white belly and under-wing. The predominantly brown female also has a white belly and under-wing, though not as extensive, as well as a white patch where the bill the face. Both sexes have a gray bill with a black tip and have distinctive yellow eyes. It derives its common name from the little drooping crest or tuft on the back of its head (Hayman and Hume, 2010 The New Birdwatcher's Pocket Guide to Britain and Europe p. 43).

Male Tufted Duck, Aythya fuligula

When feeding, it would be assumed that A. fuligula would take the largest (and most profitable) prey available. According to Draulans (1982, Journal of Animal Ecology 51 pp. 943-956) however, this is not the case. It was found that birds selected smaller mussels over larger specimens, with an increase in the selection of smaller mussels as prey density increased and depth decreased. This may seem to be contrary to the optimal foraging theory (MacArthur and Pianka, 1966 American Naturalist 100 pp. 603-609): why chose a smaller prey over a larger one? Draulans suggests that A. fuligula uses this method of feeding as smaller mussels provide less of a choking risk than large ones, the ducks may do better swallowing two small mussels in a dive rather than one large ones and larger mussels have highly variable profitabilities as larger shells have a higher likelihood of being empty.

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