A comparison of stress indicators in Gambel’s White-crowned Sparrow and California Towhee populations in natural and suburban habitats

Urbanization continues to alter natural landscapes at an accelerating rate, often threatening native biodiversity and reshaping wildlife habitats. This study aims to assess the habitat suitability of natural and suburban areas by using heterophil-to-lymphocyte ratios (H/L) and body condition as a medium-term measure of stress levels in two common Southern California songbirds, Gambel’s White-crowned Sparrow (Zonotrichia leucophrys gambelii), a migratory species found along the West Coast from Southern Oregon to California, and the California Towhee (Melozone crissalis), a year-round resident of California. We compared populations in two environments: a protected natural habitat at the Robert J. Bernard Biological Field Station (BFS), and a suburban neighborhood in Claremont, California. We predicted that both White-crowned Sparrows and California Towhees would have higher H/L ratios and lower body condition in the suburbs compared to the BFS. Field data collected in the spring and fall of 2024 included using automated cameras to observe banded birds, standard bird-banding procedures on captured birds, morphometric measurements, and blood smears that were analyzed in the lab to count for heterophils and lymphocytes. Contrary to our prediction, White-crowned Sparrow H/L ratios were lower in suburban areas than in the BFS, suggesting they experienced lower stress in the suburbs. No such effects were observed in California Towhees. In addition, body condition did not differ between sites for either species. However, White-crowned Sparrows had poorer body condition in the fall than in the spring, perhaps because of recent weight loss during fall migration. This ongoing research will explore how the expansion of urban areas affects stress and habitat quality for these species.