LIGHT AND DARK PULSES PHASE SHIFT THE CIRCADIAN LOCOMOTOR RHYTHM OF THE HOUSE SPARROW

Sparrows (Passer domesticus) are day-active birds which exhibit circadian rhythms of perch-hopping activity. The phases of sparrow's circadian activity rhythms were studied following single treatments with light ranging in duration from 1s to 30h, with single 4h dark pulses, and with doublet pulses of light and dark.;The sparrows demonstrated phase-dependent sensitivity to 4h light pulses with maximum phase advances (3.8h) at CT 20 and maximum phase delays (-1.3h) at CT 16. The sparrows also exhibited phase-dependent sensitivity to dark pulses with maximum phase advances (2.2h) at CT 16 and maximum phase delays (-0.7h) at CT 0.;The underlying pacemaker must respond to dark and/or light pulses with an immediate shift for two reasons. First, phase shifts were apparent in the first cycle following the treatment. Second, the shifts resulting from pulse doublets were predictable from the summation of individual pulses.;Phase shifts in response to all single and doublet light and/or dark treatments were predictable by considering the phase response curves. Increased light intensity and pulse duration produced larger shifts within limits.;The data are interpreted as supporting a hypothesis for entrainment to changing photoperiods which incorporates both light and dark sensitivities.