The feeding ecology of bellbirds at Craigieburn (1998)
AuthorsMurphy, Davidshow all
This thesis investigated the feeding ecology of bell birds, Anthornis melanura (Aves: Meliphagidae), at Craigieburn Forest Park to find out what, if any, aspect of the bellbirds' ecology may be limiting the pollination and possibly dispersal of two mistletoe species (peraxilla tetrapetala and Alepis flavida) in the area (shown by Ladley and Kelly, 1995b, 1996; Robertson et al., in press). Two hypotheses were tested to explain why bellbirds may not be making an adequate number of visits to mistletoe plants. The first is that there are non-mistletoe foods which are more energetically valuable to bellbirds than mistletoe fruits and nectar and so make up a larger proportion of the bellbird diet during mistletoe fruiting and flowering seasons. The second is that bellbirds concentrated on mistletoe foods when available but the numbers of bell birds at Craigiebum are too low to allow sufficient pollination and dispersal. To answer these questions, the bellbird diet, the energetic value of their food resources, and bellbird numbers were sampled over a twelve month period. Direct observation of the bellbird diet showed that they are annual generalists, on invertebrates (annual mean of 54% of bell bird diet, range 22-85%) and honeydew (annual mean of 22% of the bellbird diet, range 2-45%), and seasonal specialists, on mistletoe fruit (mean of 40% of the bellbird diet while available, range 18-60%) and nectar (mean of 39% of the bellbird diet while available, range 27-58%). The energy value of invertebrates dominated the available food energy with an annual mean of 14,255 kJ/ha (range 8,695-22,876 kJ/ha). Honeydew was the only other food source that was available all year with an annual mean energy value of 2,518 kJ/ha (range 1,199-3,283 kJ/ha). Mistletoe fruit and nectar featured prominently in the bellbird diet (18-60%) in the fruiting and flowering season even though their energy values were low with a seasonal mean energy of 2,867 kJ/ha (range 1,467-5,125 kJ/ha) for mistletoe fruit, and 3,658 kJ/ha (range 400-10,050 kJ/ha) for mistletoe nectar. This suggested that mistletoe fruit and nectar could be more valuable to the bellbirds than indicated by the measured energy values (kJ/ha) alone. One third of the variation in the bellbird diet could be explained by changing energy values of major food resources which suggests that the bellbird diet responds to the energy value of foods. As mistletoe fruit and nectar were preferred foods when in season, and the bellbird index of density was low at Craigieburn when compared to other sites, I concluded that it was the possible low number of bell birds in the area, and not their choice of diet, which was limiting mistletoe pollination, and possibly dispersal The possible low number of bell birds in the area could have been explained by the population being food limited. The time bellbirds spent foraging (feeding plus locomotion time) was a low percentage of their time budget (mean of 39%, range of 31-60%). This did not change significantly from winter to spring. If the values for late January and February were dropped, because of possible pressures from feeding nestlings, there was a significant decrease in the foraging percentage of the bellbird's time budget from winter to summer. However, when corrected for change in daylength, the hours spent foraging each day were not significantly different between seasons (mean of 4.7 hours foraging and a range of 3.6-5.7 hours) suggesting that bellbirds were not seriously limited by the availability of food. The possible low bellbird density could be a result of introduced vertebrate predators such as stoats which have had a negative impact on the populations of other native birds in New Zealand (pierce, 1993; Elliott, 1996; Elliott et al., 1996; O'Donnell, 1996; Wilson et al., 1998).