Hallingstad, Eric – Seasonal variation in immunocompetence and other physiological parameters of the American Kestrel (Falco sparverius). 2008.
Recently, the study of avian immunocompetence has evolved to include assessment of the relationships that exist between immune function and behavior, ecology, and toxicology in wild bird populations. However, few studies have investigated seasonal differences in avian immune function, particularly for migratory species. Identifying seasons that involve dampened immune response will provide essential information for effective management of migratory avian species. In this study, I quantified a component of immune strength in a migratory raptor species, the American Kestrel (Falco sparverius), during three seasons of the annual cycle. Breeding, migration, and winter season cell-mediated immunocompetence (CMI) was assessed by a cutaneous hypersensitivity response triggered by phytohemagglutinin (PHA). Baseline and stress response corticosterone (CORT) levels were collected to identify potential effects of CORT on the PHA response. Other factors included as potential predictors of the PHA response were sex, age, heterophil:lymphocyte (H/L) ratio, total white blood cell (tWBC) count, parasite abundance, a parasite intensity *tWBC interaction, hematocrit level, and body condition. Using AIC model selection techniques, the parameters included in the final model were season (winter), parasite intensity, tWBC count, and the parasite intensity *tWBC interaction. Results indicate that kestrels had dampened CMI during the winter season relative to the breeding and migration seasons. Despite likely trade-offs between the energetic demands of breeding and migration with immune response, kestrels overcame low energetic resources to maintain CMI during these seasons. This trend suggests a strategy of maintaining immune response, even at the expense of body condition, during periods of increased exposure to pathogens. The interaction effect between parasite intensity and the tWBC count suggests differing T-cell populations may have been present at high and low tWBC counts when parasite intensity was high. My results did not show a significant relationship between either baseline or stress-induced CORT levels and the PHA response. Several other physiological parameters did vary by season, including CORT responses to capture, body condition index scores, H/L ratios, parasite intensities and prevalence, and hematocrit levels. Kestrels may undergo these physiological changes in response to changing energetic demands during portions of the annual cycle.