Seasonal variation in the honeydew, invertebrate, fruit and nectar resource for bellbirds in a New Zealand mountain beech forest

To examine the seasonal availability of the major bellbird (Anthornis melanura) food sources in a mountain beech (Nothofagus solandrivar. cliffortioides) forest at Craigieburn, the invertebrate, honeydew, and mistletoe (Peraxilla tetrapetala and Alepis flavida) fruit and nectar resources were sampled over 12 months. The total available food varied 2.6-fold from a low in October (8798 kJ/ha) to a high in December (22,959 kJ/ha) with an annual mean of 15,782 kJ/ha. Invertebrates were available all year and represented 89% of the available food energy. Only 16% of the invertebrate resource was on beech foliage, and beech trunks with honeydew had 60% more invertebrate energy than trunks without honeydew. The energy value of honeydew at Craigieburn (0.9% of the total) was much lower than at lower altitude sites. The relative rankings of honeydew standing crops on 25 permanently marked trees were very constant. On an annual basis mistletoe nectar and fruit made up 6.3% and 4.9%, respectively, of total food energy, but P. tetrapetala nectar was 46% of available food in early January, and P. tetrapetala fruit was 25% of the total in March. Bellbirds spent less time foraging on invertebrates, and more time on the other foods, than energy values would predict. However, during the Peak of its short flowering season, P. tetrapetala nectar made up 46% of available energy but only 33% of bellbird foraging observations. At this site P. tetrapetala is pollen limited due to insufficient visits from pollinators. This may be because bellbirds require invertebrates for protein, or to feed to nestlings. Therefore the pollination mutualism is faltering, despite high investment in nectar by the plant.