Wildfires during fall migration affect bird movements, avian health, and migration timing in western North America: Insights from bird banding and feather isotopes

With the peak of the wildfire season in North America typically occurring during the core part of fall bird migration, migratory birds are likely to be increasingly impacted by worsening wildfire seasons during their southerly movements. In this study, we combined two approaches to assess the impact of wildfires on migratory birds captured in southern Utah. First, we used five years of bird banding data to assess how bird movement and physical condition patterns are impacted. Second, we used stable hydrogen isotopes from collected feathers to identify the geographic origin of the individuals of several migratory species in order to better understand how fires may be influencing migration. We found that when there were more wildfires active in western North America, there were more captures of birds at our banding station, likely a result of birds shifting their movements to avoid areas of fire and smoke, and that birds had worse body condition and overall health as shown from lower body masses. We also found that during periods when wildfires are especially active and severe, wildfire activity has the most significant influence on bird movements and health compared to other prominent environmental variables. Our isotopic results provide evidence that two species migration patterns varied across the year in both migration timing and likely summer origin, and show how fire activity could have affected the migratory window for some species. More broadly, our isotope data improves our knowledge of the likely geographic origins of migratory birds in this particular flyway, adds to the literature on feather-based hydrogen isotope analyses, provides some of the first published isotope data for two species, and streamlines a workflow that can aid future researchers working with feather hydrogen isotope data.