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 (Phi i) to light in the redheaded bunting (Emberiza bruniceps). Using a skeleton paradigm, we assessed the rate and magnitude of testicular resp onse as a function of the duration of an inducing light pulse and of the ti me of its introduction in Phi i. For a period of 7 weeks, birds received at an intensity of similar to 100 lux, the same (6 h) entraining light stimul us with a varied inducing light pulse: 1, 2, 4 or 6 h beginning at zeitgebe r time (zt) 11 (6L:5D:1L:12D, 6L:5D:2L:11D, 6L:5D:4L:9D or 6L:5D:6L:7D), or 1 h pulses at zt 12 (6L:6D:1L:11D) or zt 16 (6L:10D:1L:7D). The testes wer e stimulated in all LD alternations, but duration- and time-dependent effec ts of the light pulse on the rate and magnitude of the testicular response were clearly evident. Illumination of a larger portion of Phi i seems to re sult in higher rates of gonadal growth but there is a duration limit above which there will be no further increase of testicular response.