Abstract
Global‐change drivers are increasing the rates of species extinction worldwide, posing a serious threat to ecosystem functioning. Preserving the functional diversity of species is currently a priority to mitigate abrupt biodiversity loss in the coming decades. Therefore, understanding what factors better predict functional diversity loss in bird assemblages at a global scale and how existing protected areas cover the most vulnerable regions is of key importance for conservation. We examined the environmental factors associated with the risk of functional diversity loss under 3 scenarios of bird species extinction based on species distribution range size, generation length, and International Union for the Conservation of Nature conservation status. Then, we identified regions that deserve special conservation focus. We also assessed how efficiently extant terrestrial protected areas preserve particularly vulnerable bird assemblages based on predicted scenarios of extinction risk. The vulnerability of bird functional diversity increased as net primary productivity, land‐use diversity, mean annual temperature, and elevation decreased. Low values for these environmental factors were associated with a higher risk of functional diversity loss worldwide through two mechanisms: one independent of species richness that affects assemblages with low levels of niche packing and high functional dissimilarity among species, and the other that affects assemblages with low species richness and high rates of extinction. Existing protected areas ineffectively safeguarded regions with a high risk of losing functional diversity in the next decades. The global predictors and the underlying mechanisms of functional vulnerability in bird assemblages we identified can inform strategies aimed at preserving bird‐driven ecological functions worldwide.