Body masses as well as wing and tail lengths are changing for birds captured at migratory stopover sites in arid riparian regions in the Nearctic and Palearctic

Climate change is having disruptive impacts on the global environment, especially in climate-sensitive riparian zones within arid regions, which are becoming increasingly important climate refugia as surrounding areas become drier and less green. For birds, riparian zones function as critical breeding and migratory stopover sites, funneling migrants between wide-ranging breeding and non-breeding grounds. Data from such sites can inform on whether birds with diverse ranges are collectively responding to global changes, including shifts in morphology over time. 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, with a lack of studies from arid zones. In this study, we used two long-term banding datasets from arid riparian sites in the Western United States and Eastern Türkiye to assess whether these different bird communities that utilize similar habitats 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 have become heavier in the fall while the tails of birds in Türkiye have become shorter. In addition, for wing length and spring body mass, the relationships with environmental factors depended on a species’ molt strategy and migratory status. This work highlights consistent morphological trends on two different continents for bird species captured in climate-sensitive arid riparian zones.