It is not yet clear whether similar mechanisms influence the assembly of ecological communities across different continents. Here, we investigated the functional and phylogenetic diversity of bird assemblages along elevational gradients in two biogeographic regions in order to identify how these are driven by biotic factors, such as food resources and vegetation structure, and abiotic factors, such as ambient temperature and precipitation. We recorded bird species abundances on 18 and 30 plots in the Ecuadorian Andes and on Mount Kilimanjaro, respectively. We measured ten functional morphological traits, related to bird feeding and movement, and utilized bird phylogenies to compare observed values and null-model corrected effect sizes of functional and phylogenetic diversity along elevational gradients and between biogeographic regions. Furthermore, we assessed how observed values and effect sizes of functional and phylogenetic diversity were associated with the underlying gradients in available food resources, vegetation structure, temperature and precipitation. Functional and phylogenetic diversity were generally higher in species assemblages in the Ecuadorian Andes than on Mount Kilimanjaro. Both observed values and effect sizes of functional and phylogenetic diversity decreased significantly with increasing elevation in both biogeographic regions. Functional diversity consistently increased with increasing resource availability, whereas phylogenetic diversity increased with increasing vegetation heterogeneity and canopy closure in both biogeographic regions. Temperature and precipitation were not significantly associated with functional and phylogenetic diversity. Our results suggest that in both mountain systems the diversity of functional traits in bird species assemblages is the result of environmental filtering by available food resources, whereas phylogenetic diversity is primarily limited by vegetation structure. These findings suggest important differences in the main drivers of functional and phylogenetic diversity. We conclude that biotic factors might be more important for driving bird diversity patterns than abiotic factors and that a loss of resource availability and vegetation structure, e.g. through human impacts, is likely to trigger changes in community assembly on tropical mountains.
