Overview

• The study evaluated more than 3,000 bird populations and over 90,000 observations from 1950 to 2020 using counterfactual ‘no-climate-change’ models to isolate the effects of extreme heat.
• Tropical regions between 21° north and south of the equator suffered the most severe declines, with bird populations falling 25–38% as extreme-heat days rose from three annually in the mid-20th century to about 30 today.
• Heat stress is linked to higher mortality, reduced fertility, altered breeding behavior and lower juvenile survival, contributing to declines even in largely undisturbed forests.
• Passerine species exhibited the strongest heat-related population losses while Anseriformes showed more resilience, likely due to their affinity for aquatic habitats.
• Researchers warn that heat extremes pose a threat to tropical bird diversity comparable to habitat destruction and urge rapid emissions cuts alongside conservation strategies addressing thermal stress.