Foraging ecology of blue ducks Hymenolaimus malacorhynchos on a New Zealand river: Implications for conservation

Abstract

We investigated whether blue ducks Hymenolaimus malacorhynchos preferentially capture prey that have themselves become rare or that need to be present for successful re-establishment. Working at the Manganuiateao River in New Zealand, we measured the densities and relative abundances of benthic invertebrates, numbers of prey fragments in faeces of adult ducks, and foraging behaviour of adult ducks. Invertebrate densities on stones ranged from 3741 m −2 to 14,417 m −2. Stone and boulder communities were dominated by cased caddisfly larvae or Chironomidae larvae in most months. Patterns of apparent selectivity varied, but Trichoptera larvae in the family Hydrobiosidae and in the genus Aoteapsyche (Hydropsychidae) ranked highly, and cased caddis larvae consistently ranked low, in the diet. Discriminant function analysis indicated that apparent prey preferences were partly related to whether foraging blue ducks were gleaning from the tops or undersides of rocks in the river. Canonical correlation analysis showed that ingestion of stonefly and mayfly larvae was associated with diving behaviour, but it was not possible to predict the ingestion of other prey from foraging tactics. No single prey category was so highly valued by the blue ducks we studied that it might limit population establishment at new sites.