We present the first global analysis of elevational gradients in functional and phylogenetic diversity of birds and test for signals of deterministic processes in community assembly. Further, we examine for latitudinal effects in the strength of these deterministic processes.

We systematically selected, compiled and analyzed published data on bird diversity along elevational gradients. For each gradient, we calculated functional and phylogenetic diversity across elevations and described the main patterns for each diversity metric. Then, we calculated standardized effect sizes (SES) of each metric and used these SES values to (1) test the signals of environmental filtering and limiting similarity as deterministic processes shaping assemblages across elevations and (2) to compare changes in within-mountain diversity, among mountains located at different latitudes.

Birds displayed eight different patterns of functional and phylogenetic diversity across elevations, but no global pattern of increase or decrease was found. There is, however, a consistent global pattern of phylogenetic clustering, with mountain species being more closely related to each other at any given elevation. Latitude had a significant effect on within-mountain changes in functional and phylogenetic diversity across elevations, with more negative slopes (stronger decline in diversity metrics with increasing elevation) in tropical mountains.

Our findings challenge the idea that the decline of functional and phylogenetic diversity with elevation is a general pattern, emphasizing the uniqueness of each mountain system. In spite of this great variability, we found a latitudinal effect in the patterns of within-mountain functional and phylogenetic dispersion of birds after controlling for effects of species richness. Environmental filtering, thus, may act differently in tropical and temperate mountains, and calls for more comparative studies on the mechanisms driving community assembly at different latitudes.