Glucocorticoid hormones allow individuals to rapidly adjust their physiology and behavior to meet the challenges of a variable environment. An individual’s baseline concentration of glucocorticoids can reflect shifts in life history stage and resource demands while mediating a suite of physiological and behavioral changes that include immune modulation and resource allocation. Thus, glucocorticoids could facilitate a response to parasites that is optimized for an individual’s specific challenges and life history stage. We investigated the relationship between endogenous circulating glucocorticoids and measures of resistance and tolerance to Haemosporidian parasites (including those that cause avian malaria) in red-winged blackbirds (Agelaius phoeniceus). We found that higher endogenous concentrations of circulating glucocorticoids were associated with reduced costs of parasite infection, which is indicative of higher tolerance, but were unrelated to parasite burden in free ranging, breeding male birds. Post-breeding, both males and females with higher glucocorticoid concentrations had higher measures of tolerance to Haemosporidian infection. Our findings suggest a potentially adaptive role for glucocorticoids in shifting the response to parasites to align with an individual’s current physiological state and the challenges they face
We studied adult, breeding red-winged blackbirds at two sites in southeastern Ontario, Canada: Queen’s University Biological Station (44°34′02.3″ N, 76 °19′28.4″ W) and outside the city of Kingston (44°15′04.8″ N, 76 °28′43.6″ W). Red-winged blackbirds breeding in this area have high prevalence of Haemosporidian parasites. In the late 1980s and early 1990s, prevalence as detected on blood smears ranged between 30 and 56% in females and 35–71% in males (Weatherhead, 1990, Weatherhead and Bennett, 1991, Weatherhead et al., 1993). However, actual prevalence might have been higher, as detection on blood smears tends to be lower than with PCR (Garamszegi, 2010). Between 2013 and 2015, Haemosporidian prevalence as detected by PCR was over 90% (Supplementary Material, Table S1).
We prepared blood smears from each sample by spreading a drop of blood (approximately 5–10 μL) on glass slides, and immediately air-dried the smears and fixed each in absolute methanol. We stained smears with Diff-Quick (IMEB INC., San Marcos, CA, USA) within four months of sample collection.
Blood was initially stored on ice (for no more than 6 h) before being centrifuged at 6000 RPM for 10 min. After centrifugation, we measured hematocrit for the first two capillary tubes collected from each bird using a microhematocrit card. The average of the two hematocrit measures was used in the final analysis. We separated the plasma and red blood cells and stored both at −20 °C until analysis.