Stable understory microclimates within undisturbed rainforests are commonly regarded as refugia against climate change. Yet, this assertion has rarely been tested, and contrasts with pervasive population declines observed for many bird species in Amazonian forests unaffected by landscape change or direct exploitation. We tested if population vulnerability of resident rainforest birds may stem from climatic effects, particularly during dry seasons with elevated temperatures and reduced rainfall. Data from 4264 individual bird captures collected over 27 years reveal strong negative consequences of harsher dry-season conditions on survival for 27 of 29 species; an increase of 1° C reduced the aggregate survival of the understory bird community by 34%. These results link climate change to demographic processes in the heart of Amazonian rainforest, raising doubts about the capacity of pristine rainforests to protect their unparalleled biodiversity under increasingly severe climate change.

Constant effort mist net data from the KM41 site in the Biological Dynamics of Forest Fragmentation Project (BDFFP). Each bird banding transect hosted a line of 16 mist-nets (NEBBA-type ATX, 36 mm mesh, 12x2.5 m), with nets set with the bottom at ground level and opened from 0600 to 1400 for a single day of sampling. All captured birds were banded with uniquely numbered aluminum bands. We mistnetted birds between May through October, from 1985 to 2012.