Acute phase responses of passerine birds: characterization and seasonal variation

The acute phase response (APR) is the first line of defense that many vertebrates employ during a pathogenic challenge. This response is composed of a suite of physiological, behavioral, hormonal, and metabolic changes that include fever, iron sequestration, anorexia, adipsia, somnolence, and activation of the hypothalamo-pituitary-adrenal (HPA) axis and suppression of the hypothalamo-pituitary-gonadal (HPG) axis. Although well-studied in mammals and domesticated birds, the APR of passerines is virtually unexplored. Here, we characterize the APR in several species of Emberizidae and examine seasonal variation. Captive and free-living sparrows were treated with lipopolysaccharide (LPS), an immunogenic agent that triggers the APR without actually causing infection. LPS treatment activates the HPA axis, suppresses the HPG axis, decreases activity and food and water intake, and induces short-term hypothermia in captives, as well as inhibiting territorial aggressive behavior and song in free-living males. The magnitude of the APR also varies seasonally in males, implicating a tradeoff between physiological processes within particular life-history stages, such as reproduction. The proximate mechanisms underlying this seasonal modulation may include hormonal suppression by the steroid testosterone and seasonal differences in energy stores, which are rapidly depleted to a minimum body mass threshold as a result of APR-induced sickness behavior. We conclude by comparing this variation in APR to seasonal variation of avian stress responses.