Drivers of Variation in Avian Community Composition Across a Tropical Island Montane Elevational Gradient

Elevational variation in avian community composition can be significantly influenced by habitat degradation, fragmentation and secondary forest growth. Few studies have identified the drivers of changes in bird community composition across disturbed montane elevational gradients of smaller tropical islands. We examined patterns of avian diversity using long-term mist-net data (2008–2014) across three different forest elevations along a tropical montane elevational gradient in the Northern Range of Trinidad, West Indies. All three sites (lowland mature secondary forest, mid-elevation highly disturbed secondary forest, and undisturbed high elevation forest) were found to have distinctive bird communities. Turnover rather than nestedness explained most of the total dissimilarity between sites. Whilst some turnover could be attributed to elevation, changes to diversity at the mid-elevation site result more from local habitat heterogeneity related to human activities and secondary growth, with increased species richness attributable to habitat-generalist species indicative of disturbance. Significant anti-nestedness in species occupancy was observed, underpinned by the loss of ground-dwelling and understory insectivores from the mid-elevation site. Differences in bird community composition, in contrast, were driven by the abundance of specialist nectarivores in the highest elevation undisturbed montane forest, and by generalist nectarivores and frugivores at lower elevations.