Time Course of Sensitivity of the Photoinducible Phase to Light in the Redheaded Bunting, Emberiza bruniceps

This study investigated the differential sensitivity of the photoinducible phase (Φi) to light in the redheaded bunting (Emberiza bruniceps). Using a skeleton paradigm, we assessed the rate and magnitude of testicular response as a function of the duration of an inducing light pulse and of the time of its introduction in Φi. For a period of 7 weeks, birds received at an intensity of ~100 lux, the same (6 h) entraining light stimulus with a varied inducing light pulse: 1, 2, 4 or 6 h beginning at zeitgeber time (zt) 11 (6L:5D:1L:12D, 6L:5D:2L:11D, 6L:5D:4L:9D or 6L:5D:6L:7D), or 1h pulses at zt 12 (6L:6D:1L:11D) or zt 16 (6L:10D:1L:7D). The testes were stimulated in all LD alternations, but duration- and time-dependent effects of the light pulse on the rate and magnitude of the testicular response were clearly evident. Illumination of a larger portion of Φi seems to result in higher rates of gonadal growth but there is a duration limit above which there will be no further increase of testicular response.