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

Abstract

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.

• Bunting
• Sparrow
• Body Mass
• Nanda–Hamner LD‐Cycle
• Circadian
• Photoperiodism
• Testes