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

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.