The Effect of Urbanization and Sampling Date on Blood Immunophysiology and Haemosporidian Parasitemia in Cardueline Finches

Urbanization alters habitat structure, resource availability, and influences pathogen transmission, leading to widespread ecological disruptions as it continues to expand. We investigated the impact of urbanization on physiological stress and vector-borne parasite prevalence in 3 Cardueline finch species: house finches (Haemorhous mexicanus), lesser goldfinches (Spinus psaltria), and pine siskins (S. pinus). We hypothesized that urbanization level (urban, suburban, and rural) would be associated with higher heterophil to lymphocyte (H/L) ratios, a proxy of stress, and increased parasite prevalence. To test this, we quantified H/L ratios, parasite prevalence and parasite load on blood smears with light microscopy. We found no significant effect of urbanization on H/L ratios, suggesting that urbanization did not strongly influence this measure of stress. Instead, sampling date was a significant predictor of H/L ratios, with H/L increasing as the season progressed, which may reflect the breeding season in California. Sampling date also significantly predicted parasitemia in house finches, suggesting a potential role of seasonal variation in infection risk. Also, suburban house finches had significantly higher parasite prevalence than urban individuals, which may reflect differences in environmental factors such as nearby water availability and agricultural land use. Across species, lesser goldfinches exhibited significantly higher H/L ratios than house finches, but pine siskins did not differ from the other finches. Our results emphasize the importance of considering both seasonal dynamics and local environmental conditions when assessing the ecological impacts of urbanization on avian health.