New Zealand Journal of Ecology (1997) 21(2): 169- 179

Honeyeaters and the New Zealand forest flora: The utilisation and profitability of small flowers

Research Article
Isabel Castro  
Alastair W. Robertson  
  1. Department of Ecology, Massey University, Palmerston North, New Zealand

New Zealand flowers are frequently considered unspecialised allowing easy access to pollen and nectar by a wide range of visitors. Most conform with a syndrome of insect pollination (entomophily). Pollination of forest flowers by birds has been described for a range of species whose flowers are morphologically ornithophilous. On Kapiti Island and Little Barrier Island, all three species of New Zealand honeyeaters have been described feeding on flowers currently assumed to be entomophilous or where the pollination system is unknown. The persistence and regularity of visits suggests that the birds are obtaining suitable rewards in the form of nectar and could be serving as pollinators. We measured the nectar energetic value from flowers of three ornithophilous and five entomophilous species. Nectar production over 24 hours was highest in ornithophilous species, but the standing crop of nectar overlapped—ornithophilous species: Metrosideros fulgens (standing crop 6.6 J), Metrosideros excelsa (22 J), and Fuchsia excorticata (1.8 J); and entomophilous: Pittosporum crassifolium (23 J), Pseudopanax arboreus (1.5 J), Dysoxylum spectabile (3.7 early flowers -6.7 J late flowers), Pittosporum eugenioides (2.7 J) and Geniostoma rupestre (1.8 J). The entomophilous species present the flowers in aggregation and as result birds can visit a large number flowers per minute. We found that the average estimated nectar consumption rate for all the entomophilous species except G. rupestre was enough to sustain the two smaller New Zealand honeyeaters (hihi energy requirements= 0.12 kJ min(-1), median energy obtained: 0.16 kJ min(-1) D. spectabile—0.57 kJ min(-1) P. crassifolium); bellbird energy requirements = 0.10 kJ min(-1), median energy obtained: 0.14 kJ min(-1) D. spectabile—0.68 kJ min(-1) P. crassifolium). However, we estimate that if the birds are able to selectively forage on the flowers with most nectar, the energetic returns of all species may be sufficient for hihi and bellbird (hihi: 0.18 kJ min(-1) G. rupestre—0.93 kJ min(-1); P. crassifolium; bellbird: 0.12 kJ min(-1) G. rupestre 1.11 kJ min(-1) P. crassifolium). If tui (energy requirements: 0.25 kJ min(-1), forages randomly, only P. crassifolium (0.80 kJ min(- 1)) and D. spectabile late in the season (0.30 kJ min(-1)) provide sufficient returns, but if selective, P. arboreus (0.45 kJ min(-1)) may also suffice. We suggest that because (a) the nectar produced by entomophilous flowers provides sufficient energy to sustain the energetic requirements of birds, and (b) these plants flower in the cooler months when insect activity is reduced, birds might have played a wider role in pollination than previously considered. This finding is of particular importance because the abundance of New Zealand honeyeaters on the mainland has decreased considerably since human colonisation and this could be affecting forest regeneration.