Invertebrate Food Supplies and Diet of Blue Duck on Rivers in Two Regions of the North Island, New Zealand

Benthic invertebrates and samples of blue duck faeces were collected in September 1988 from sites along Manganuiateao River, central North Island, and in November 1988 from seven rivers and streams on the East Cape. The occurrence of invertebrate taxa in the faeces varied within and between rivers, and within pairs of birds and family groups on the East Cape. In both regions, most blue duck had been consuming large proportions of cased caddisfly larvae. These are thought to have been mainly species of Helicopsyche and Pycnocentrodes at the East Cape sites and Beraeoptera roria at the Manganuiateao sites. Plecoptera larvae were also relatively abundant in blue duck faeces from most Manganuiateao sites in September. Overall, blue duck consumed proportionately more cased caddisfly larvae than occurred in the benthos (especially at the East Cape sites), but fewer Chironomidae, Coloburiscus humeralis and leptophlebiid mayfly (mainly Deleatidium spp.) larvae. Factors that affect the type of invertebrate foods available to blue duck at a particular site could include habitat heterogeneity, chance encounter, frequency and magnitude of floods, and geographic differences in the pool of invertebrate colonists. Apparent selectivity or avoidance of some benthic invertebrate groups by blue duck may partly reflect predator evasion by fast-moving invertebrate species, and differences in activity and distribution on upper stone surfaces where invertebrates should be more susceptible to predation by blue duck.