Abstract
The importance of light as a regulating factor of the sexual cycle in birds has been demonstrated by Rowan 1 (junco), Bissonnette 2 (European starling), Miyazaki 3 (Zosterops palpebrosa) and Kirschbaum 4 (English sparrow). These investigators have observed a precocious development of the testes following daily increases in the period of illumination. Such experiments were usually carried out during the early winter. The present author has undertaken to determine whether lengthened daily periods are equally effective at a time further removed from the normal breeding season. In this paper, a group of male sparrows, exposed to additional light beginning November 18th, is compared with another group first exposed on September 30th.
The November group of sparrows had been in the laboratory 16 days previous to the artificial lengthening of the days and all possessed the horn-brown bill of the sexually quiescent period. The birds received 4 1/2 additional hours of light, daily, from a 100-watt incandescent bulb throughout the experiment, increasing the day to a total of about 14 hours. The effect of such treatment both on bill color and testis is shown in Table I.
The acceleration of testicular development which was observed in the birds of this group is in full agreement with the results of the aforementioned authors.
A second group of male sparrows, however, (Table II) first subjected to a lengthened light period on September 30th, gave a decidedly different result. These birds had been in the laboratory 10 days prior to experimental use. The light period was extended 2 hours the first day, 3 hours the second and from this time on 15 additional minutes daily until, after 14 days, the period had been increased 6 1/2 hours, making a total of approximately 17 hours of light.
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