Body masses as well as wing and tail lengths are changing for birds captured at arid riparian migratory stopover sites

Climate change is having disruptive impacts on global biodiversity, especially in climate-sensitive riparian zones within arid regions, which are becoming increasingly important climate refugia as surrounding areas become drier and less green. Birds are increasingly exhibiting climate-driven changes in body size and shape, including increasing wing length and declining body mass. However, these trends are variable around the world, and there is a lack of studies from arid zones. In this study, we used two long-term banding datasets from climate-sensitive arid riparian sites in the Western United States and Eastern Türkiye to assess whether these different bird communities that utilize similar habitats on different continents have experienced consistent changes in their morphology over the past couple decades. We also examined whether morphology was related to environmental variables (e.g., temperature, precipitation, landscape greenness), and whether these environmental relationships varied depending on species’ movement, molt, or dietary ecology. Our study finds that, over time, bird species from both continents became heavier in the spring and have been developing longer wings, resulting in greater relative wing length. Additionally, birds captured in Utah are trending heavier in the fall while the tails of birds in Türkiye have become shorter. Furthermore, for wing length and spring body mass, the relationships with environmental factors depended on a species’ molt strategy and migratory status, respectively, with increased temperatures resulting in shorter wings for birds that molt on their breeding grounds, and increased vegetative greenness resulting in heavier masses for migratory birds. Notably, our results provide only limited support for thermoregulatory hypotheses (e.g., Allen’s and Bergmann’s Rules) as the underlying causes of morphological change and instead point toward resource availability as a potential driver of tends in body mass, wing length, and tail length for birds from both continents.