LAUSR

AIM: We evaluated the influence of climate on the structure of bird communities along precipitation gradients. We hypothesize that mechanisms responsible for community turnover along precipitation gradients are similar to mechanisms operating along temperature and latitudinal gradients. We tested the hypothesis that environmental conditions affect community composition in dry forests, whereas biotic interactions affect community composition in wet forests. LOCATION: Low‐elevation forests along a precipitation gradient in Colombia where precipitation ranges from 700–4,000 mm annually but elevation and temperature remain constant. TIME PERIOD: Present day. MAJOR TAXA STUDIED: Neotropical forest birds. METHODS: We sampled at 291 points in nine study areas (localities) across the c. 3,000‐mm precipitation range. In each locality, we obtained climatic characteristics and phylogenetic, morphological and physiological proxy data to test predictions about the evolutionary relationships and distribution of traits. RESULTS: Bird communities changed abruptly along the precipitation gradient and differed between dry and wet forests. Analyses of phylogenetic relationships, trait space, and observations at nests suggested that environmental filtering is more important in dry than in wet forests, especially for breeding. In contrast, we found little evidence that competition was more important in wet than in dry forests. Nest predation or competition for nest space, however, may be more critical in wetter forests. CONCLUSIONS: The two distinct bird communities we documented suggest that lowland precipitation gradients, across which temperature is constant, can be as important as temperature gradients in generating high beta diversity. We conclude that breeding in bird communities might be crucial for determining community assembly along environmental gradients. Given that recent population declines in tropical birds have been attributed to changes in precipitation, by understanding the mechanisms underlying community assembly along precipitation gradients our study may improve our ability to understand those declines and predict the effects of climate change on Neotropical avifauna.