Shape‐shifting in relative wing length of juvenile shorebirds: no evidence of developmental temperatures driving morphological changes

Morphological changes concurrent with climate change are increasingly identified in birds, often through decreasing body size and increasing appendage size. Such changes could have thermoregulatory implications, through the improved surface area to body ratio they provide. Due to the role of bird wings in thermoregulation, wing length relative to body mass may be changing as another form of shape‐shifting, where increased relative wing length may facilitate increased heat loss as climates warm. We investigated changes in relative wing length on a dataset of nearly 20 000 juvenile shorebirds from 11 species over the past four decades, to determine changes in morphology and whether these are linked to developmental temperatures. Overall, across species, we found that relative wing length increased across the 43‐year study period in populations migrating to tropical northern Australia but not in those migrating to temperate southern Australia. Furthermore, we found that changes in relative wing length were not driven by immediate responses to high temperature at the breeding ground during juvenile growth. These results may suggest that relative wing length increases occur in shorebirds occupying already warm climates, where they might potentially be more thermally challenged under further warming, but that such changes are not occurring through plastic mechanisms during development.