Circadian rhythms of perch-hopping activity were recorded for over 200 days from pairs of house sparrows (Passer domesticus) kept in the same cage and treated with a light- dark cycle (LD 12:12), with constant light (LL), and with constant dark (DD). Most pairs entrained to the LD 12:12 cycle and exhibited free-running circadian rhythms in LL and DD. Two rhythms were visible in the records of 40% of the pairs in DD and in 50% of the pairs in LL. Otherwise, free-running pairs displayed a single circadian rhythm of perching activity.
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