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Glucocorticoid stress hormones are important for energy mobilization as well as regulation of the immune system, and thus these hormones are particularly likely to both influence and respond to pathogen infection in vertebrates. In this study, we examined how the glucocorticoid stress response in house finches (Haemorhous mexicanus) interacts with experimental infection of the naturally-occurring bacterial pathogen, Mycoplasma gallisepticum (MG). We also investigated whether infection-induced concentrations of corticosterone (CORT), the primary glucocorticoid in birds, were associated with the expression of sickness behavior, the lethargy typically observed in vertebrates early in infection. We found that experimental infection with MG resulted in significantly higher CORT levels on day 5 post-infection, but this effect appeared to be limited to female house finches only. Regardless of sex, infected individuals with greater disease severity had the highest CORT concentrations on day 5 post-infection. House finches exposed to MG exhibited behavioral changes, with infected birds having significantly lower activity levels than sham-inoculated individuals. However, CORT concentrations and the extent of sickness behaviors exhibited among infected birds were not associated. Finally, pre-infection CORT concentrations were associated with reduced inflammation and pathogen load in inoculated males, but not females. Our results suggest that the house finch glucocorticoid stress response may both influence and respond to MG infection in sex-specific ways, but because we had a relatively low sample size of males, future work should confirm these patterns. Finally, manipulative experiments should be performed to test whether the glucocorticoid stress response acts as a brake on the inflammatory response associated with MG infection in house finches.

Animals inhabiting urban areas must simultaneously cope with the unique challenges presented by this novel habitat type while exploiting the distinctive opportunities it offers. The costs and benefits of urban living are often assumed to be consistent across time, but may in fact vary depending on the habitat features influencing them. Here we examine the glucocorticoid levels and body condition of song sparrows (Melospiza melodia) resident at urban and rural sites over four consecutive years to determine whether these traits, which may be linked to the relative costs and benefits of these respective habitats, are consistent over time. Glucocorticoid levels and body condition varied by year in both habitat types. While habitat alone did not influence glucocorticoid levels, there was a significant interaction between year and habitat, indicating that glucocorticoids differ between habitats in some years but not others. There was no discernable effect of habitat alone on body condition. Overall, these data suggest that the costs and benefits of inhabiting urban versus rural habitats differ substantially from year to year.

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