Climate change is having widespread impacts on ecosystems worldwide, but the extent to which it may have already altered the abundance of major taxa remains unknown. Here, we show that the historical intensification of climate extremes has substantially reduced the abundance of terrestrial bird populations over the past five decades. Combining over 86,000 time-series observations of population abundance with exposure to locally defined climate extremes, we find strong and robust evidence that hot extremes reduce abundance growth, dominating the role of average temperature conditions or changes in precipitation. These impacts are largest in lower-latitude tropical regions and are robust when controlling for changing human pressure and the geographic and taxonomic bias of observed populations. Quantifying the portion of extreme intensification which is attributable to climate change implies substantial historical reductions in bird-abundance of up to 50% at low latitudes. These results provide new evidence that human-driven intensification of heat extremes is degrading the abundance of a major terrestrial vertebrate class, of particular relevance given hitherto unexplained declines of tropical birds in undisturbed habitats.