Abstract: Studies by Rowan (1930 and preceding papers) and Bissonnette (1931, a, b, 1932, a, b, and preceding papers) and Bissonnette and Wadlund (1931) have shown that in juncos, canaries, crows and starlings, at least, among birds, the sexual cycle is conditioned to a great degree by daily light ration. Sexual activity in both males and females can be modified at will by altering the daily light ration with controlled electric lighting from ordinary incandescent bulbs. Gonad changes, controllable in this way, are closely correlated with migrations in juncos and crows, as demonstrated by Rowan (1930, and preceding papers), who, however, maintains that it is the added periods of muscular exercise, rather than the light, as such, that condition these changes, in spite of the fact that it is always the added light he uses, rather than exercise, to get his effects. Bissonnette disagrees, and holds it is the light that is the major factor in conditioning the changes in sexual cycles. It has been found, also, that intensity and colour, or wave-length, as well as daily period of light are factors in conditioning sexual activity in starlings, and the method of increase of light also plays a part (Bissonnette, 1931, b, 1932 ; Bissonnette and Wadlund, 1931).