Testing the microclimate hypothesis: Light environment and population trends of Neotropical birds

Species vary in their vulnerability to extirpation or extinction in response to habitat loss, including deforestation. A variety of correlates of vulnerability have been found, but few empirical data have been gathered that provide insight into potential causation. We attempted to elucidate cause via a test of the “microclimate hypothesis” at two sites in the northern Neotropics. We censussed birds at 234 points that varied in distance from a deforested edge to >2 km into a forest. At each of these points we recorded or calculated ten environmental and microclimate variables that allowed us to tie environmental data to bird assemblages directly. We found that canopy cover increased, wind decreased, and ambient temperature, vapor pressure density, and heat index varied little or not at all with distance from a forested edge. Yet light penetration was the key variable, as the light environment was affected within 50 m of an edge, and many species were associated with low-light conditions. Crucially, on the basis of regional population trends, species in low-light environments are more vulnerable to extirpation. Our results provide the clearest support to date of the hypothesis that sensitivity to light is a key factor limiting occupancy of birds in fragmented tropical forests and lend support to at least this aspect of the microclimate hypothesis.