FEEDING BEHAVIOUR AND ECOLOGY OF NEW GUINEA RAINFOREST INSECTIVOROUS PASSERINES

Fifty species of insectivorous warblers Sylviidae, flycatchers Muscicapidae and whistlers Pachycephalidae were studied in primary rainforest at various localities in New Guinea. The structure of the various forest types is described and the birds' feeding ecology and behaviour analysed by recognizing three main foraging techniques and five horizontal and three vertical basic structural divisions of the habitats. Altitudinal ranges of the species are assessed to determine potential co-existence and they are divided into lowland and lower montane groups (either side of the main avifaunal discontinuity at 1500 m) with a third small group occurring in both areas and a fourth group of 12 lower montane species that occur also in the structurally much simpler Upper Montane forest. The feeding behaviour and ecology of the species within each major habitat are compared, with particular attention to taxonomically related and ecologically similar species. Other important considerations—additional behavioural differences, notable morphological distinctions, altitudinal separation of ranges within the habitat—are also noted. The likely importance of differences in foraging behaviour and feeding sites for reducing competition between related species is amply demonstrated, members of several pairs and groups of species have nearly mutually exclusive preferences. The overall pattern of habitat utilization is, however, extremely complex with nearly all stations used, in a variety of ways, by several species and there are many instances of substantial similarity between pairs of species, often involving congeners. The calculation of information theory derived indices of foraging diversity and overlap enables more general comparisons between the altitudinally graded habitats to be made and differences related to current ideas on tropical species diversity. Between Lowland and Lower Montane forest there is a fairly general trend of reduction in foraging diversity and decrease in the mean overlap between species in many genera and groups. The 12 species that continue into the simpler Upper Montane forest show very significantly reduced foraging diversity (compared with their values in lower montane forest) and also less overlap, indicating a different relationship between these species in the absence of the other Lower Montane forest birds. Together these results suggest that the most tropical (i.e., lowland) species show greatest overlap but do not necessarily have smaller niches. In progressively higher habitats there is a bias to the disappearance of generalist (high diversity index) species. These mainly use flycatcher-gleaning techniques supporting suggestions that the increase in insectivorous species in the tropics is partly due to exploitation of feeding strategies related to hovering. Habitat and ecological factors influencing this are assessed. The importance of altitudinal isolating mechanisms is also discussed and, amongst the species studied, both on average and in specific cases, those with the greatest similarities in foraging behaviour and ecology are segregated altitudinally and do not co-exist. It is suggested, however, that substantial overlap between many co-existing tropical species may not be abnormal, but rather an adaptation for ensuring maximum efficiency of habitat utilization in the prevailing environmental conditions of tropical rainforest.