Differential responses of the photoperiodic clock in two passerine birds possessing a strongly self-sustained circadian system.

To investigate whether the photoperiodic clocks of species possessing strongly self-sustaining circadian clocks share identical features, we compared the full response cycle (initiation and termination of the response) in body mass and testes of the non-migratory house sparrow (Passer domesticus) with that of the migratory redheaded bunting (Emberiza bruniceps) under Nanda-Hamner experiments. Birds were exposed to a 36 h day (L:D=6:30 h), controls exposed to a 24 h day (L:D=6:18 h), for a period of 31 weeks. By week 18 of L:D=6:18 h, there was a small increase in body mass among sparrows, but not among buntings, and the testes of bunting did not grow, while those of sparrow grew slightly. The response to L:D=6:30 h is of particular interest. There was a rapid gain and subsequent loss in the body mass of bunting, but not of sparrows. Further, both species underwent a testicular cycle as if they were exposed to long days, but the response of sparrows was slower and hence delayed the attainment of peak testicular size. Such a differential response to exotic light cycles between these two photosensitive species, despite their similar circadian oscillatory properties (strong self-sustainment), could suggest a species-specific adaptation of the endogenous clock involved in photoperiodic regulation of avian seasonality.