External and Internal Factors in Sexual Activity

There is a seasonal cycle of sexual activity for all species of birds. It is more or less pronounced according to the species, and it is primarily controlled by the external factor of light, the daily duration and intensity of which vary during -the year, the minimum amount of light being present on December 21, while the maximum is reached on June 21 in the northern hemisphere. Rowan54 was the first to demonstrate in a scientific manner that the artificial light of electric bulbs stimulated the gonads in the junco. He thought that the light acted indirectly, increasing muscular activity and metabolism, which secondarily stimulated the gonads.55 Bissonnette" ' 23, 24 first demonstrated in the starling "that it is light per se, and not muscular exercise, which when increasing causes the progressive changes in the testes. . .,,2' This fact was confirmed by several authors for the fowl,40 51, 38, 58 sparrow,41 59' 39 mourning-dove,33 Zosterops,47 pheasant and quail,29 blue-jay,28 and duck.5' 6 Working with the starling, Bissonnette showed that lights of different wave-lengths stimulate the testes to varying degrees; red and white lights stimulate25 while green light,25' 26. 27 and perhaps also violet light do not.27 In the immature duck, when equal quantities of radiant energy are given, the growth of the testes is very marked with red light, less pronounced with yellow-green light, and scarcely observalble with green and blue lights.'2 A preliminary report of the effects of lights of different colors was published earlier.2" The completed experiments will be presented in this paper. In the turkey no stimulation was caused by blue and blue-green lights, according to Scott and Payne.56 In the sparrow, Ringoen52 noted that "red light is vastly more effective than green." In the starling, Burger3" found that blue-green may promote some sexual stimulation and that "only wave-lengths of approximately 0.58 and