FORAGING ECOLOGY OF BLUE DUCKS HYMENOLAIMUS-MALACORHYNCHOS ON A NEW-ZEALAND RIVER - IMPLICATIONS FOR CONSERVATION

We investigated whether blue ducks Hymenolaimus malacorhynchos prefere ntially capture prey that have themselves become rare or that need to be present for successful re-establishment. Working at the Manganuiate ao River in New Zealand, we measured the densities and relative abunda nces of benthic invertebrates, numbers of prey fragments in faeces of adult ducks, and foraging behaviour of adult ducks. Invertebrate densi ties on stones ranged from 3741 m(-2) to 14,417 m(-2). Stone and bould er communities were dominated by cased cacldisfly larvae or Chironomid ae larvae in most months. Patterns of apparent selectivity varied but Trichoptera larvae in the family Hydrobiosidae and in the genus Aoteap syche (Hydropsychidae) ranked highly, and cased caddis larvae consiste ntly ranked low, in the diet. Discriminant function analysis indicated that apparent prey preferences were partly related to whether foragin g blue ducks were gleaning from the tops or undersides of rocks in the river. Canonical correlation analysis showed that ingestion of stonef ly and mayfly larvae was associated with diving behaviour, but it was not possible to predict the ingestion of other prey from foraging tact ics. No single prey category was so highly valued by the blue ducks we studied that it might limit population establishment at new sites.