Photostimulation of Japanese quail by dim light depends upon photophase contrast, not light intensity.

Three experiments were conducted to determine whether dim light is interpreted by Japanese quail as subjective day or night, and whether this interpretation depends upon absolute light intensity. Birds were exposed to 24-h days consisting of either bright light (2500-3000 lx) with dim light (0.5-5 lx) or dim light with darkness. Locomotor activity was higher in the brighter photophase, whether it was bright light or dim light, indicating that the birds interpreted the brighter phase as daytime. Dim light produced daytime activity levels when paired with darkness, but it produced nighttime activity when paired with bright light, indicating that activity rhythms are determined by relative not absolute light intensity. Similarly, photostimulation, as measured by growth of the cloacal protrusion area (CPA), depended upon photic context, not absolute light intensity. CPA growth occurred when birds were exposed to 16 h of dim light with 8 h of darkness (16dm:8dk) but not when exposed to 10 h of bright light with 14 h of dim light (10bt:14dm). Constant dim light was stimulatory regardless of previous dim light context. Photostimulation appears to depend upon subjective interpretations of day and night rather than solely upon light intensity.