Geographic redistributions are insufficient to mitigate the erosion of species environmental niches

As climate change accelerates, species may survive in place thanks to niche plasticity or adaptation or must redistribute to conserve their environmental niches1. Examples of such geographical range shifts abound2-4, but to date an assessment of species success in retaining niches and limiting their climate change exposure is missing. Here, we develop a novel method to account for biases inherent in tens of millions of citizen science observations, allowing us to evaluate how species have mitigated their climatic niche loss using geographical redistributions. We find that over 20 years, 384 North American bird species shifted their summertime distributions 0.73{degrees} north, mitigating their expected exposure to warming by [~]1.16 {degrees}C and thus averting 44% of expected niche loss had they stayed in place. Despite these movements, species were still exposed to an average [~]1.47 {degrees}C increase in temperature and few species achieved complete niche retention. Meanwhile, species only mitigated [~]0.47 {degrees}C (11%) in winter, shifting their niches by [~]3.74 {degrees}C, with almost no species fully retaining their niches. Species moving the furthest north succeeded most in conserving niches across both seasons. As expected, but previously untested at this scale, species that have physiological characteristics associated with dispersal achieved the greatest redistributions and niche retention. Most geographical redistributions have only been partially effective towards mitigating climate change and the gap between climate change exposure and species historical niches is growing rapidly even in a highly mobile group such as birds, raising concerns about the ability of less mobile taxa to persist in a warmer world.