Two Circadian Rhythms in Pairs of Sparrows

Abstract

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 lightdark 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.In this report, we describe and discuss studies of the circadian rhythms of house sparrows (Passer domesticus) housed in cages in pairs, and compare them with rhythms of sparrows caged individually. In the experiments reported here, we measured rhythms of individual sparrows and of pairs of sparrows in a light-dark cycle (LD 12:12), constant light (LL), and constant dark (DD). METHODSThe methods we used for recording rhythms of sparrows and for data analysis were the same as in our prior investigations (Binkley and Mosher, 1987). We note that the recording method has limitations. First, total activity was not measured, only perching activity. Second, when a pair of birds was in a cage, only one event record was obtained from each cage, so the record showed the two birds' collective (and inseparable) activity on the perches. It was possible in this situation for one bird to &dquo;mask&dquo; the activity of the other bird by dominating the perches. For the LL experiment, the lamps were partially covered with black electrical tape to reduce light intensities to 40-240 lux so that a larger percentage of birds would free-run (Binkley, 1977).The birds were subjected to sequences of treatments (Table 1) consisting of lighting regimens (LD 12:12, DD, and LL) and social opportunities (paired or singly housed). The recording obtained during one lighting-social experiment regimen for one bird or one pair is called a &dquo;record&dquo; for purposes of the analyses here. Each bird contributed one to four such records to the study. Records in which there were technical problems with the lights or recorder, or where activity was too sparse to make an evaluation, or during which a bird died, were not used; however, a few records that were shorter or longer than those obtained from the standard protocols were included in the analyses. Data analyses were made by making measurements on the sparrows' ink event