Circadian versus circannual rhythm in the photoperiodic programming of seasonal responses in Eurasian tree sparrow (Passer montanus).

Experiments were performed on the subtropical tree sparrow (Passer montanus) to investigate whether day length, as a proximate factor, involves circadian rhythm in timing seasonal responses or these events are programmed by the mediation of an endogenous circannual rhythm. The experiments began from the two equinoxes, i.e. March and September, and continued for about 18 months. In each equinox, one group of wild birds of both sex was exposed to near simulated day length as it varies in nature at Shillong (Latitude 25°34'N, Longitude 91°53'E), and the other group was given a photoperiodic treatment in the reverse order corresponding to natural variation in day length from the other equinox. The birds exposed to near simulated day lengths in March and September showed gonadal, bill colour and molting responses, as observed in the wild birds in nature. Moreover, when photosensitive birds of March were given photoperiodic treatment in the reverse order (i.e., natural variation in day lengths beginning from September), they maintained their photosensitivity and responded only when they received increasing day length in the following September. However, when the photorefractory birds of September were exposed to reverse photoperiodic treatment (i.e. natural day length as it varies from March), they terminated their photorefractoriness after receiving decreasing day length in March the next year and responded when they again received increasing day length in the following September. These results strongly suggest that photoperiodic regulation of seasonal responses in the tree sparrow involves an endogenous circadian rhythm and exclude the possibility of circannual rhythm generation.