Alexandra McQueen, Marcel Klaassen, Glenn J Tattersall, Sara Ryding, Robyn Atkinson, Roz Jessop, Chris J Hassell, Maureen Christie, Arkadiusz Fröhlich, Matthew R. E. Symonds

Animals are predicted to shrink and shape-shift as the climate warms; declining in size, while their appendages lengthen. Determining which types of species are undergoing these morphological changes, and why, is critical to understanding species responses to global change, including potential adaptation to climate warming. We examine body size and bill length changes in 25 shorebird species using extensive field data (>200,000 observations) collected over 46 years (1975-2021) by community scientists. We show widespread body size declines over time, and after short-term exposure to warmer summers. Meanwhile, shorebird bills are lengthening over time but shorten after hot summers. Shrinking and shape-shifting patterns are consistent across ecologically diverse shorebirds from tropical and temperate Australia, are more pronounced in smaller species, and vary according to migration behaviour. These widespread morphological changes could be explained by multiple drivers, including adaptive and maladaptive responses to nutritional stress, or by thermal adaptation to climate warming.