Variation in baseline corticosterone levels of Tree Sparrow (Passer montanus) populations along an urban gradient in Beijing, China

Abstract

Rapid urbanization is a major anthropogenic pressure on bird species that rely on vegetation for food and shelter. Since the baseline corticosterone concentration (BCC) in some bird species has been found to increase slightly in response to environmental challenges, we hypothesized that urbanization could also induce an increase in BCC. To test this hypothesis, we compared the BCC of Tree Sparrow (Passer montanus) populations in five urban and two rural habitats and analyzed the relationship between BCC and the degree of urbanization. Here, we show that the BCCs of Tree Sparrow populations were strongly and positively correlated with the degree of urbanization of the habitat. Average BCC of Tree Sparrows from high-rise residential areas and a university campus were significantly higher than those from rural areas, suggesting that Tree Sparrows living in highly urbanized areas have greater environmental challenges than those in rural areas. However, the average BCC of birds from an urban park was not significantly different to that of birds at rural sites and also differed significantly from that of birds at two of the other urban sites. These results suggest that urbanization could pose environmental challenges for Tree Sparrows, a species that appears relatively well-adapted to human-modified environments. The marked variation in BCC between different urban sites indicates that conclusions drawn from data collected at single sites must be interpreted with caution.