The Significance of the Dark Period in the Photoperiodic Response of Male Juncos and Whitethroated Sparrows

Experiments were conducted with slate-colored juncos and white-throated sparrows to test the thesis that the dark period plays a definitive role in the photoperiodic response of these birds.2. The results of the experiment using male juncos were as follows:(a) Testes of birds on a short day (10 hours of light, 14 hours of darkness) underwent little or no development during the 8 weeks of the experiment.(b) Exposure of other juncos to a long day (16 hours of light, 8 hours of darkness) during this same period resulted in their testes attaining full breeding condition.(c) Experimental birds were placed on a light schedule which consisted of 8¼ hours of light followed by a dark period, the middle of which was interrupted by a second light period of 1¾ hours; thus they received a total (as in (a)) of 10 hours of light per 24-hour period, but dark periods were only 7 hours long. By the end of 8 weeks considerable testicular development had occurred among all males placed on this "interrupted-night" schedule. The testes of four birds had developed mature sperm (late stage VI or VII); the other two were somewhat less developed.3. The response of white-throated sparrows placed under these same conditions was similar to that of the juncos.4. The results show clearly that in the photoperiodic response of these birds there is a critically important dark-dependent phase. This dark-period dependence appears to be characteristic of all photoperiodic responses, in both plants and animals.