Photostimulation of blackheaded bunting: subjective interpretation of day and night depends upon both photophase contrast and light intensity.

Blackheaded buntings were exposed to constant dim light (approximately 2 lux) for 77 days and subsequently to 12:12 illumination, consisting of either dim light:complete darkness (dl:dk) (2:0 lux) or bright light:dim light (bl:dl) (10, 35, and 90 lux:2, 10, and 35 lux, respectively, with 180 degrees bl:dl transitions) for 2-3 weeks each. Birds were unstimulated until day 100, under dim LL (dl:dl) and 12dl:12dk. Photostimulation with a significant increase (p less than 0.001) in average body weight and testis volume occurred on exposure to 12bl:12dl (10:2 lux); birds remained fat and testes were enlarged until the end (day 173) of the experiment. Locomotor activity also did not have a discernible pattern under the nongonadostimulatory photoregimes. Birds became active in the bright photophase (subjective day) of 12bl:12dl (10:2 lux) on day 5 (day 105 of the experiment), but beginning on day 113 more intense perch-hopping was initiated in the dim photophase (subjective night), indicating the appearance of nocturnal migratory restlessness in the caged birds. A 180 degrees transition in the bl:dl photophases at this time for 2 weeks resulted in complete shift in the timing of daytime and nighttime activity. This trend is maintained under all subsequent 12bl:12dl exposures. These results suggest that the light intensity required for photostimulation in the blackheaded bunting is absolute, and that the subjective interpretation of day and night depends upon the photophase contrast, as well.