The Relationships between Energetic Condition, Immune System Cellular Components, Testosterone Corticosterone, and Hemoparasites in Breeding Birds

When energy resources are low, redirecting energy away from the immune system is one way a bird can compensate, though short-term benefits could have long-term consequences if the immune system is chronically energetically compromised. Testosterone and corticosterone may negatively affect immune system function when persistently present, while hemoparasites could potentially deplete a bird\u27s energy reserves. Song sparrows (Melospiza melodia) and Yellow warblers (Dendroica petechia) were captured in mist-nets during the breeding season of 2004 (May-July) to explore the relationships between energetic condition, the immune system, testosterone, and corticosterone. Body mass and the metabolites free glycerol and triglyceride were used as indicators of energetic condition and heterophil to lymphocyte ratio and eosinophil count as measures of immune system function. Testosterone and corticosterone concentrations were determined in Sorig sparrows. Female Song sparrows were implanted with testosterone proprionate to explore the effects of chronically high testosterone levels on these variables. The effects of parasitic infection were examined for a variety of birds. Free glycerol and hematocrit decreased in Yellow warblers during the study period. Corticosterone affected the mass of male Song sparrows, while glycerol affected female mass. Eosinophil count increased with glycerol concentration in Song sparrows and increased with triglyceride concentration for Yellow warbler males. Song sparrows with high heterophil to lymphocyte ratios had higher triglyceride concentrations than birds with low ratios and Yellow warblers with high heterophil to lymphocyte ratios had higher masses. Testosterone concentration decreased as triglyceride concentration increased in male Song sparrows. Corticosterone and testosterone increased together in male Song sparrows. Mass decreased as corticosterone increased in Song sparrows. Testosterone implants did not significantly affect any of the variables studied. Birds in this study had a higher prevalence of Trypanosomes than birds at similar latitudes in other studies. Males were parasitized more than females. The presence of hemoparasites did not affect the energetic trajectory of Song sparrows or Yellow warblers. Both glycerol and triglyceride concentrations correlated with condition index and immune system cellular components. Differences in life history between the two species may account for differences in the factors affecting energetic condition and the immune system. The sexes of different species may have more in common with each other than with members of the opposite sex of the same species due to the differing roles of the sexes during the breeding season. Energetic condition was correlated with the components of the immune system. Testosterone and corticosterone may both need to be present to produce changes in energetic condition and the immune system. High Trypanosome prevalence may be due to the study\u27s close proximity to hemoparasite vectors. Greater male exposure to parasite vectors may lead to higher infection rates. Hemoparasitic infections may drain immune system resources or resurge when resources are low