Persistence of circannual cycles in a migratory bird held in constant dim light

Abstract

1. Most studies of long-term endogenous cycles in birds have been performed in light:dark (L:D) cycles. In this study, male dark-eyed juncos (Junco hyemalis) were held in constant dim light (1−3 lx) and constant temperature for up to 3 years. Testicular cycles, molt, migratory fattening and nocturnal migratory activity (Zugunruhe) were monitored continuously. 2. Birds exhibited up to 3 cycles of gonadal growth and regression, spring pre-migratory fattening, molt, and spring and fall Zugunruhe. The annual cycle events maintained appropriate sequential relationships. The amplitude of successive testicular cycles did not decrease. 3. The annual cycles of individual birds drifted out of phase with one another, i.e., they appeared to free-run. Cycle lengths were quite variable: 6 to 20 months based on the interval between testicular maxima, and from 6 to 21 months based on the interval between the onset of post-nuptial molts. The cycles had a mean duration of 13.7 months (SD = 4.2 months) between testicular maxima, and 15.0 months (SD = 3.8 months) between the onset of post-nuptial molts. Therefore they may be considered circannual. 4. These data provide the strongest evidence for the existence of endogenous, free-running long-term rhythms in birds. They are consistent with a hypothesis in which annual cycles are based on a self-sustaining circannual oscillator upon which seasonal changes in daylength act as a Zeitgeber.