Wing and tail lengths are changing for birds in arid riparian regions in the Nearctic and Palearctic

The effects of climate change are having increasingly disruptive impacts on the global environment, especially on climate-sensitive arid riparian zones that are becoming increasingly important climate refugia for organisms like birds as surrounding areas become drier and less green. In this study, we used two long-term banding datasets from arid riparian locations in both the Western United States and Eastern Türkiye to assess whether these bird communities have been changing their morphology in the past couple decades and to compare the direction of these trends between two widely different sets of birds at both the community and species levels. Notably, our study finds that birds from both continents have been developing longer wings over time, and the tails of birds in Türkiye are getting shorter, but birds have not been undergoing changes in their body mass or overall size. This work is the first joint assessment of morphological trends for birds in both the New and Old World, the first from climate-sensitive arid riparian zones, and the first study to examine trends in tail length. We also examined the impact and influence of environmental factors and shared biological traits on these trends among birds in our study, and found that a species’ molt strategy, migratory status, and primary diet had notable impacts on the relationships between environmental variables and wing length. We surmise how factors at the population and species levels may be helping drive morphological changes, while underscoring that there is much still to learn about the mechanisms influencing morphology.