Ongoing changes in the avifauna of La Selva Biological Station, Costa Rica: Twenty-three years of Christmas Bird Counts

•Analysis of 23 years of Christmas Bird Count data from La Selva Biological Station.•Contrary to previous studies, diet was not associated with population change.•Habitat and body size best predicted population change.•Smaller birds declined, suggesting a physiological mechanism influencing declines.

Tropical forest fragmentation influences community composition via differential species-level effects. Avian responses to fragmentation at La Selva Biological Station are, in part, responsible for the particular concern over the fate of understory insectivorous species. However, since the 1990s, much previously deforested land within and surrounding La Selva has reverted to forest, providing an opportunity to test hypotheses explaining ongoing avifaunal change. Analyses of 23 years (1989–2011) of Christmas Bird Counts reveal that 63 of 202 species have increased whereas 44 are declining, with declines occurring more rapidly than increases. Habitat association was an important predictor of population trends, as understory birds continue to decline whereas forest generalists increased. Our results differ from previous work in the tropics by revealing that, at La Selva, insectivores are not currently suffering greater declines than birds of other dietary guilds. Instead, body size was more strongly associated with population change than was diet, with smaller birds having more negative population trends than larger birds. These results suggest that we must consider additional hypotheses that may explain ongoing population declines of tropical birds. In particular, the associations between population trends and body size implicate physiological mechanisms influencing population change, which may result from direct or indirect consequences of changing climates.