Changes in the daily rhythm of plasma corticosterone concentration related to seasonal conditions in the white-throated sparrow, Zonotrichia albicollis.

Circadian variations in concentrations of plasma corticosterone were investigated in the white-throated sparrow maintained on short (10-hr) or long (16-hr) daily photoperiods. In addition, the plasma concentrations of corticosterone were determined throughout a day in birds that were in the reproductively photosensitive spring migratory condition, the reproductively photorefractory post nuptial molt condition, and the fall migratory condition. Distinct unimodal rhythms were found in photosensitive birds. The daily rise occurred 12 hr after the offset of light in birds kept on both the short and the long photoperiodic regimens. There was no discernible daily variation in photorefractory birds kept on a 16 hr daily photoperiod and there was a bimodal rhythm in the birds that were in the fall migratory condition. The results are consistent with an hypothesis that assigns an important role to the circadian rhythm of corticosteroid concentration in the photoperiodic mechanism controlling seasonal reproductive and migratory conditions in the white-throated sparrow.