Abstract
Birds are widely expected to shrink as the climate warms, but few tropical studies span decades. In the central Amazon, mean body mass fell in all 77 species studied and a third of species lengthened their wings. We analysed 24 years (1995–2018) of morphology from 73 passerine species (12,571 live-capture records; ATLANTIC BIRD TRAITS) across the Brazilian Atlantic Forest, a region that warmed by about 0.25°C per decade. The records came from 47 research programmes whose sites changed over time, so we modelled contributor and municipality explicitly and checked by simulation that these models recover trends planted in the real sampling structure. Unadjusted models suggested a steep wing-length decline and increases in bill width and trait variability. Once sampling turnover was modelled, the wing decline was less than half as steep (−1.0% per decade, 95% credible interval spanning zero), and the bill-width and variability changes disappeared. Body mass declined by 0.9% per decade, close to the mean Amazonian rate of 1.0%, but its interval spanned zero (−2.8 to +1.0%). Responses varied strongly among species, and most species showed no change distinguishable from zero. Wings did not lengthen, and among individuals measured for both traits, mass tended to be retained relative to wing length. Neither trait was associated with interannual temperature anomalies. Atlantic Forest passerines thus show neither the consistent mass loss across species nor the wing elongation reported from the Amazon, although their average mass change is compatible with the Amazonian rate. Compiled trait databases can extend long-term monitoring to under-studied tropical regions, but only when sampling structure is modelled: ignoring it here would have more than doubled the estimated wing decline.