William D. Newmark

Of the myriad responses to climate change, an emerging trend is the widespread decrease in animal body size with warming temperatures. Birds, in particular, have been shown to be decreasing in body size in several areas – most notably the Amazon Basin and temperate North America – but trends in much of the world remain unexplored. Here, we analyze temporal trends and climatic associations of body mass for 42 resident bird species over 36 years in the Usambara Mountains of Tanzania, a tropical montane global biodiversity hotspot. In addition, we assess trends in wing length and mass:wing ratio over 21 years. Finally, we examine whether species‐specific trends in body mass are related to average body size or to trends in apparent survival. Overall, species' average body mass in the Usambara Mountains increased by 0.023 g decade ‐1 , amounting to increases of 4.1% over 36 years. These long‐term shifts in body mass were strongly and positively associated with annual mean temperature but showed no relationship with precipitation. Wing length increased on average by 2.0% over a 21‐year period, yet there was mixed evidence for trends in mass:wing ratio, suggesting that body size in general is increasing. While percentage trends in body mass were not related to trends in apparent annual survival, smaller species did display greater proportional increases in body mass. Although the drivers of increased body size remain unclear – and climate change cannot be ruled out – such increases among Afrotropical montane birds provide an intriguing counterpoint to observed declines in body mass elsewhere and suggest that trends in body mass in tropical birds may be mediated by biogeography or other abiotic or biotic drivers.