Physiological responses to photoperiod in three cardueline finch species

Cardueline finches (canaries, goldfinches, and rosefinches, etc.) vary widely in the degree to which their natural reproductive schedules track seasonal changes in photoperiod. In this study, we tested for photo-induction of reproductive development and photorefractoriness in males of three cardueline finch species: pine siskins (Carduelis pinus), common redpolls (Carduelis flammea), and white-winged crossbills (Loxia leucoptera). Exposure to long days (20L:4D) in winter induced gonadal growth and elevation of circulating luteinizing hormone (LH) in all three species. After 4.5 months on constant long days, gonadal regression was complete in redpolls and siskins, but only partial in crossbills. Feather molt was most advanced in redpolls, slightly less advanced in siskins, and least advanced in crossbills. These results indicate that the reproductive systems of all three species were stimulated by long days, but that the crossbills, which are temporal opportunists, either did not become absolutely photorefractory, or developed refractoriness more slowly than did the other two species. Reproductive development of controls held for 4.5 months on constant short days (5L:19D) was negligible in redpolls and crossbills, but substantial in siskins, suggesting that of the three species, pine siskins may be the least dependent on long days for reproductive development. Changes in fat deposition and body mass also differed among species. Short day redpolls tended to be fatter and heavier than long day redpolls, and long day crossbills tended to be fatter and heavier than short day crossbills. Body mass and fat depot of siskins remained high irrespective of photoperiod. These results illustrate substantial variation among these close relatives, and are consistent with the idea that differences in apparent reproductive flexibility among cardueline taxa relate to interspecific differences in responsiveness to environmental cues, not simply to differences in the environments experienced. This kind of information is critical to an understanding of the mechanistic bases of natural variation in reproductive schedules, and of how different species may be affected by modifications to the environment.