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Plasma corticosterone, adrenal mass, winter weather, and season in nonbredding populations of Dark-eyed Juncos (Junco hyemalis hyemalis)

Plasma levels of corticosterone (B) have been measured in free-living Dark- eyed Juncos (Junco hyemalis hyemalis) wintering in Michigan, Indiana, and Tennessee. Total adrenal dry mass also was determined for a large number of wintering juncos collected at these three locations. All populations had access to supplemental food. When Michigan, Indiana, and Tennessee winter populations were compared, plasma B was significantly greater when new snow had fallen on trap days (midwinter) than when it had not (early winter). However, the relative importance of a direct response to proximate snowfall and a seasonal change as independent causes of this pattern could not be determined. In the Tennessee population in early January, plasma B was greater on two days with new snowfall than on two days without new snowfall. Total adrenal dry mass was significantly and positively correlated with recent snowfall, possibly in support of elevated plasma B on snowy days. Together with studies showing increased fat reserves shortly after new snowfall in the Dark- eyed Junco, these results suggest that corticosterone might be involved in directing increased feeding and associated lipogenesis as inclement weather sets in. Although a direct response to snowfall is suggested, factors correlated with snowfall (e.g. barometric pressure) also may cause elevated titers of B during inclement weather. Received 14 October 1991, accepted 4 March 1992.