Aim:We examined whether the available surface area, temperature, or habitat complexity (foliage height diversity index) determine species richness of birds (and species richness of individual feeding guilds) along a complete forest elevational gradient. Further, we focused on the relationship between species richness of insectivorous birds and the availability of their food resources. Location: Elevational gradient (200-3,700 m) of Mt Wilhelm (4,509 m a.s.l.), Central Range, Papua New Guinea. Taxon: Birds. Methods:We collected data on bird communities at eight sites (500 m elevational increment) during three surveys encompassing both dry and wet seasons over a 2-year period. We used point counts, mist-netting, and random walks throughout a standardized area. We tested three predictors of diversity and all of their combinations, in conjunction with sensitivity analyses for spatial effects. Habitat complexity (foliage height diversity index) and temperature were locally measured; surface area available within 200 m elevational intervals was obtained using GIS software. We further locally surveyed insect biomass and related it to species richness of insectivorous birds.Results: Birds displayed a monotonic decline in species richness (from 113 to 35 bird species) with increasing elevation, and a nested pattern of species loss. The observed patterns were best explained by habitat complexity for the insectivores, frugivoreinsectivores, and total number of bird species. The available surface area was the best predictor for frugivorous birds. The mean temperature had a high correlation with species richness of all birds and gave the best fit of species richness for insectivore-nectarivores and pure nectarivores. The biomass of insectivorous birds correlated with the biomass of arthropods. We ruled out the possibility that the elevational pattern observed in birds could be driven by a single phylogenetic radiation. Main conclusions:We observed species richness patterns correlate well with habitat complexity and mean temperature, but mean temperature was not ranked as high as expected. Our results thus challenge the generally expected high importance of temperature as a regulator of water availability, production, and biochemical process that influence species richness, and underscore the importance of vegetation structure and the food resources as the driver of observed species richness.

Food preferences and exploitation are crucial to many aspects of avian ecology and are of increasing importance as we progress in our understanding of community ecology. We studied birds and their feeding specialization in the Central Range of Papua New Guinea, at eight study sites along a complete (200 to 3700 m a.s.l.) rainforest elevational gradient. The relative species richness and abundance increased with increasing elevation for insect and nectar eating birds, and decreased with elevation for fruit feeding birds. Using emetic tartar, we coerced 999 individuals from 99 bird species to regurgitate their stomach contents and studied these food samples. The proportion of arthropods in food samples increased with increasing elevation at the expense of plant material. Body size of arthropods eaten by birds decreased with increasing elevation. This reflected the parallel elevational trend in the body size of arthropods available in the forest understory. Body size of insectivorous birds was significantly positively correlated with the body size of arthropods they ate. Coleoptera were the most exploited arthropods, followed by Araneae, Hymenoptera, and Lepidoptera. Selectivity indexes showed that most of the arthropod taxa were taken opportunistically, reflecting the spatial patterns in arthropod abundances to which the birds were exposed.

Tropical forests worldwide are being fragmented at a rapid rate, causing a tremendous loss of biodiversity. Determining the impacts of forest disturbance and fragmentation on tropical biotas is therefore a central goal of conservation biology. We focused on bird communities in the interior (>100 m from forest edge) of forest fragments (300, 600, and 1200 ha) in the lowlands of Papua New Guinea and compared them with those in continuous forest. We surveyed bird communities using point counts, mist‐netting, and random walks, and measured habitat and microclimate characteristics at each site. We also surveyed leaf‐dwelling arthropods, butterflies, and ants, and obtained diet samples from birds to examine food availability and food preferences. We recorded significantly fewer bird species per point in the 300‐ha forest fragment than in other study sites. Overall, we recorded 80, 84, and 88 species, respectively, in forest fragments, and 102 in continuous forest. Frugivores (especially large frugivores) and insectivores had lower species richness in forest fragments than continuous forest. Our results did not support the food scarcity hypothesis, that is, the decline of insectivorous birds in forest fragments is caused by an impoverished invertebrate prey base. We also found no significant differences among forest fragments and continuous forest in microclimates of forest interiors. Rather, we found that microhabitats preferred by sensitive birds (i.e., 30% of species with the strongest preferences for continuous forest) were less common in forest fragments (19%–31% of points) than in continuous forest (86% of points). Our results suggest that changes in microhabitats may make forest fragments unsuitable for sensitive species. However, limited dispersal capabilities could also make some species of birds less likely to disperse and occupy fragments. In addition, impoverished food resources, size of the forest fragment, or hunting pressure could contribute to the absence of large frugivorous birds in forest fragments. The forest fragments in our study, preserved as village‐based protected areas, were not large enough to sustain the bird communities found in continuous forest. However, because these fragments still contained numerous bird species, preservation of such areas can be an important component of management strategies to conserve rainforests and birds in Papua New Guinea.