THE SIGNIFICANCE OF THE DARK PERIOD IN THE PHOTOPERIODIC RESPONSE OF MALE JUNCOS AND WHITETHROATED SPARROWS

Abstract

In expts. to test the thesis that the dark period plays a definitive role in the photoperiodic response of birds, the following results were obtained with male juncos (J. hyemalis): (a) on a short day (10 hrs. of light, 14 hrs. of darkness) the testes underwent little or no development during the 8 weeks of the experiment; (b) on a long day (16 hrs. of light, 8 hrs. of darkness) testes attaining full breeding condition; (c) on a short day with interrupted night (8 1/4 hrs. of light followed by a dark period the middle of which was interrupted by a second light period of 1 3/4 hrs.[long dash]thus a total of 10 hrs. light per 24-hr. period, but dark periods only 7 hrs. long) considerable testicular development occurred; testes of four birds had developed mature sperm (late stage VI or VII) two others were somewhat less developed. White-throated sparrows responded similarly. These results are interpreted as indicating 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.