Acoustical Entrainment of Circadian Activity Rhythms in House Sparrows: Constant Light is not Necessary

Cycles of presence/absence of social factors can sometimes entrain the circadian activity rhythms of diurnal animals kept under constant light. This might represent a special case of photic entrainment; social factors might awaken the subjects and make them see the light at times when they are normally unaware of it because of sleep. The present study attempted to falsify this hypothesis by entraining the activity rhythms of house sparrows (Passer domesticus) kept in constant darkness. The zeitgebers used were daily 2‐h playbacks of conspecific vocalizations, and daily 1.5‐h acoustical (non‐conspecific) disturbance. 7 birds were tested; four (all males) entrained to both stimuli, while the other three (all females) mostly showed relative coordination. Entrainment took place when the stimuli coincided with the late part of the birds' normal sleep time. No anticipatory activity preceded the daily stimulus presentations. In a second experiment, 8 more male sparrows in constant darkness began their free‐running activity earlier (phase‐advanced) on days that followed the presentation of a single 2‐h bout of acoustical disturbance in the last half of their normal sleep time. Stimulus presentations at other times had no effect. Opening the eyes and seeing the light are not necessary for acoustical (and social) entrainment, but other physiological consequences or correlates of sleep interruption may be involved.