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.
Study species and population
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).
During May – June 2013, we captured 32 female and 28 male birds on their territories using mist nets between 5:00 and 11:00 AM. We lured males into nets with playback of conspecific song and calls accompanied by a taxidermied mount of a conspecific, and we flushed females from their nest into the net. We individually marked each bird with a numbered aluminum band from the Canadian Wildlife Service and a unique combination of color bands to allow individual identification. From each individual we measured body mass (to the nearest 0.5 g) with a Pesola spring scale, tarsus length (to the nearest 0.1 mm) with calipers, and wing chord (to the nearest 0.5 mm) with a wing rule.
We collected blood samples from each individual to measure: 1) corticosterone levels, 2) the presence/absence and burden of avian Haemosporidians by genus (Plasmodium, Haemoproteus, and Leucocytozoon), 3) hematocrit, and 4) polychromasia. Quickly after capture, we punctured the brachial vein with a 26 gauge ½ inch needle and collected ∼350 μL of blood into heparinized capillary tubes. We recorded the time between capture and the completion of blood sampling, and only samples collected within three minutes of the bird entering the net were included in analyses (Romero and Reed, 2005). In samples collected within three minutes, we found no significant relationship between corticosterone concentrations and either time between capture and blood sampling or the time between net set-up and capture for males or females (all P ≥ 0.18, N = 43), therefore, we considered these samples to be representative of baseline corticosterone. We measured baseline corticosterone in 25 females and 18 males.
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.