Differential migration and an endocrine response to stress in wintering dark-eyed juncos (Junco hyemalis)

The dark-eyed junco (Junco hyemalis) exhibits differential migration in aut umn that, in general, results in females overwintering south of males, and young within each sex overwintering north of older birds. Individuals overw intering at higher latitudes face less predictable and more challenging env ironmental conditions. Rapid increases in circulating levels of the energy- regulating glucocorticosteroid, corticosterone, occur in response to enviro nmental stressors. To establish whether the strength of acute corticosteron e secretion was correlated with the probability of encountering poor enviro nmental conditions, we compared the corticosterone stress response (e.g. in itial plasma concentrations at the time of capture and 30 min later) in dar k-eyed juncos overwintering in Mississippi (MS), USA, near the southern lim it of their wintering range, with juncos overwintering in New York (NY), US A, near the northern limit of their wintering range. During two winters, 22 males and one female were sampled in NY; 13 males, 12 females and one bird of undetermined sex were sampled in MS. Not unexpectedly NY birds carried greater fat reserves that resulted in a significantly higher value of energ etic condition (mass corrected for wing cord cubed). There was no differenc e between the two winters sampled at either site, nor was there an effect o f sex on patterns of corticosterone secretion in MS birds. With sexes poole d, MS and NY birds had similar baseline corticosterone levels. However, as predicted, NY birds exhibited significantly higher corticosterone concentra tions 30 mill after capture. These results support the hypothesis that bird s wintering in less predictable, more extreme environments show a higher am plitude corticosterone response, which may enable them to adjust their beha viour and physiology more rapidly in response to environmental stressors su ch as storms. Adrenocortical sensitivity may be a part of the physiological milieu associated with differential migration in juncos whether it results from endogenous differences in the migratory programmes of individuals or from acclimatization to local environmental conditions remains to be determ ined.