Scarce or distracted? Bellbird (Anthornis melanura) foraging and diet in an area of inadequate mistletoe pollination

Recent work at several central South Island sites has shown that the bird-pollinated mistletoe Peraxilla tetrapetala (Loranthaceae) is extensively pollen-limited. We studied the diet, time-budget, and densities of its principal pollinator, bellbirds (Anthornis melanura, Meliphagidae), at Craigieburn to find out what aspect of bellbird ecology may be limiting pollination. Direct observations of bellbird diets showed that they are annual generalists on invertebrates (diet range 22-85% of food items) and honeydew (diet range 2-45%), and concentrate seasonally on mistletoe fruit (18-60%) and mistletoe nectar (27-58%) when available. The bellbirds at Craigieburn are more insectivorous than New Zealand's other two honeyeaters (tui and stitchbirds). In general, bellbirds are most similar to the short-billed guild of Australian honeyeaters in their beak morphology, foraging behaviour, and diet choice, but with a greater importance of fruit in the bellbird diet. The annual mean number of bellbirds recorded per 5-minute count (1.05) at Craigieburn was relatively low, even compared to other eastern South Island sites, which have lower counts of bellbirds than the western South Island and offshore islands. As mistletoe fruit and nectar were preferred foods when in season, and bellbird counts were low at Craigieburn, we conclude that it is the probable low number of bellbirds in the area, and not their choice of diet, which limits mistletoe pollination and dispersal. The bellbird population at Craigieburn did not appear to be food limited as bellbirds spent less than 20% of their time feeding, and the number of hours per day bellbirds spent feeding and foraging did not change significantly from winter to summer while food resources became more plentiful. Other pressures that limit the bellbird population size, particularly predation from introduced mammals, would appear more likely explanations for poor pollination and disperser services to mistletoes at Craigieburn.