Large reductions in tropical bird abundance attributable to heat extreme intensification.

Although species exhibit widespread sensitivity to environmental conditions, the extent to which human-driven climate change may have already altered their abundance remains unclear. Here we quantify the impact of climate change on bird populations from across the world by combining models of their response to environmental conditions with a climate attribution framework. We identify a dominant role of intensified heat extremes compared to changes in average temperature and precipitation. Increased interannual exposure to hot extremes reduces annual abundance growth rates most strongly in lower-latitude tropical regions, with effects robust when controlling for changing human industrial pressure and other long-term drivers. Compared to a counterfactual without human-driven climate change, the historical intensification of heat extremes has caused a 25-38% reduction in the level of abundance of tropical birds, which has accumulated from 1950 to 2020. Across observed tropical bird populations, impacts of climate change have typically been larger than direct human pressure, the opposite across sub-tropical regions. Overall, these results showcase how human-driven climate change is already reshaping biodiversity globally and may explain reported declines of birds in undisturbed tropical habitats.