Spectral composition of light as a Zeitgeber for birds living in the high arctic summer.

The hypothesis was tested whether periodic changes in spectral composition of light are an effective zeitgeber for synchronization (entrainment) of circadian rhythms of birds breeding in the high arctic summer. Two palearctic passerine birds, the brambling (Fringilla montifringilla) and the common redpoll (Carduelis f. flammea), which both breed during summer at latitudes above the Arctic Circle and migrate to temperate zone latitudes for wintering, were studied. To investigate the effect of daily alterations in light spectrum, bramblings were tested under 24-h cycles of fluorescent lights of different color temperatures (12 h at ca 5,000 K and 12 h at ca 2,500 K) and energy ratios varying between 16:1 and 0.3:1. Entrainment of activity rhythms occurred at energy ratios of 1:1 (in all seven birds tested) and 0.8:1 (in three of seven birds tested) with activity mostly within the "blue" phase (ca 5,000 K). Light cycles alternating between narrow spectral ranges of blue (max. at 440 nm) and red (max. at 650 nm) and energy ratios of 1:1 and 1.1:1 (equivalent to ratios of photons flux of ca 0.67:1 and 0.73:1, respectively) were also effective in entraining the bramblings' activity rhythms (8 and 9 of 11 birds, respectively) with activity occurring primarily during the red phase. The activity rhythms of most common redpolls tested also entrained to blue:red light cycles (B:R 16.5:7.5 h) with equal energies during the blue and red phase (equivalent to a photons flux ratio of ca 0.67:1). The results support the hypothesis that passerine birds breeding in the arctic during summer may use the daily changes in spectal composition of sunlight as a zeitgeber for synchronizing physiological and behavioral rhythms, if changes in light intensity are not a reliable environmental cue.