Data from: The biotic interactions hypothesis partially explains bird species turnover along a lowland Neotropical precipitation gradient

Aim: We evaluated the influence of climate in determining bird communities along precipitation gradients. We argue that mechanisms responsible for community turnover along precipitation gradients are similar to mechanisms operating along temperature and latitudinal gradients. We test 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 – 4000 mm annually but elevation and temperature remain constant. Time period: Present day Major Taxa Studied: Tropical Forest Birds. Methods: We performed 291 bird counts in nine study areas across the ~3000 mm precipitation range. In each locality we obtained climatic characteristics, and a phylogenetic, morphological and physiological proxy data set to test predictions about the evolutionary relationships and distribution of traits. Results: Bird communities changed abruptly along the precipitation gradient and could be divided into dry and wet forest communities. Analyses of phylogenetic relationships, trait space, and observations at nests suggested that environmental filtering is more important in dry forest, especially for breeding. In contrast, we found little evidence that competition was more important in wet forest. 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, where temperature is constant, can be as important as temperature gradients in generating high beta diversity. We conclude that the breeding process 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