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. Prior research utilizing bird banding data has shown that birds are fundamentally changing their morphology and therefore overall body shape, though these trends have not entirely been uniform or consistent in direction or magnitude and seemingly vary depending on contextual factors like geography and the bird community. 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 birds in Türkiye have been shortening their tails, but birds overall have not been undergoing changes in their body mass or size. This work is the first joint assessment of morphological trends for birds in both the New and Old World, and the first from climate-sensitive arid riparian zones, as well as the first study to examine trends in tail length. We also sought to examine the impact and influence of environmental factors and shared biological traits on these trends among birds in our study, such as rising temperature and summer molting grounds of species for wing length. We surmise about how factors at the population and species levels may be helping drive morphological changes, though we also underscore that there is much still to learn about the mechanisms influencing morphology.