Changes in plasma corticosterone and adrenocortical response to stress during the breeding cycle in high altitude...

Plasma corticosterone levels were monitored in a breeding population of Dusky Flycatchers (Empidonax oberholseri) at Tioga Pass, in the eastern Sierra Nevada. Seasonal changes in baseline plasma corticosterone levels were largely related to changes in reproductive status. Levels in both sexes were highest during the period preceding the female's initiation of a clutch. Females, alone, incubated, but males provided food regularly for their incubating mates. Excepting transient peaks in female corticosterone levels that immediately preceded ovipositions, steep declines in baseline levels of corticosterone in both sexes corresponded to the onset of parental attentiveness. Serial measurements of plasma corticosterone levels over a 1-h period of restraint, indicated that these declines originate at the level of the hypothalamo-pituitary-adrenal response system. Increases in stress-induced plasma corticosterone were significantly greater during the period preceding clutch initiation (prenesting) than during the nesting period (incubation and nestling periods). Despite the changes in corticosterone between prenesting and nesting periods, variability in corticosterone levels during both stages was negatively correlated with body condition (body mass and fat, corrected for size), and level of parental investment. Highest levels of plasma corticosterone were observed in lighter, leaner flycatchers, and during the hours when self-foraging activities were highest; lowest levels were seen in heavier, fatter birds, and those foraging for chicks. This relationship between corticosterone and parental care extended to a small number of accessory males that were acting as helpers at the nest. These results suggest that variation in the length of the reproductive cycle and degree of parental investment may help to explain the level of adaptive modulation of the adrenal stress response in species that breed in unpredictable environments.