Similarly, experiments with the common junco or snowbird reported 

 by Rowan (1931 : 121), resulted in increased development of the sexual 

 organs by the end of December, although the birds were confined 

 in outdoor aviaries in Canada, and had been exposed to temperatures 

 as low as — 44° F. From the first of November until early in January, 

 the juncos were subjected to ever-increasing light, supplied in the 

 aviaries by electric bulbs. As regards illumination, they were thus 

 artificially provided with conditions approximating those of spring. 

 At the close of this period, it was found that the sexual organs of the 

 birds had attained the maximum development normally associated 

 with spring. With gradual reduction of the lighting over a period 

 of little more than i month, the organs returned to their normal winter 

 condition. 



After a consideration of all evidence, including the fact that no ultra- 

 violet rays were used, it was concluded that the explanation lay in the 

 increased exercise taken during the periods of increased light. A 

 simple test whereby certain birds were forced by mechanical means 

 to take more exercise, the light being so reduced that there was merely 

 sufficient glow for them to see the advancing mechanism that forced 

 them into movement, showed that the rate of development of the 

 sexual organs exactly paralleled that in the birds that were exposed 

 to extended periods of illumination in the outdoor aviaries. Other 

 features in this experiment — such as the behavior of the birds them- 

 selves — also indicated that more activity due to increased light is the 

 governing cause of the spring development of the sexual organs. If 

 this development be accepted as a controlling cause of migration, then 

 this experiment must be recognized as of great importance. 



Upon closer analysis, however, it is found that this theory, like those 

 before discussed, is open to serious objections. First, some of our 

 summer residents that migrate south for the winter do not stop in 

 equatorial regions, where they might find the periods of day and night 

 about equally divided, but push on beyond, some penetrating as far 

 south as Patagonia. Also it might be asked : "If the lengthening day 

 is the stimulating factor, why should our summer birds wintering in 

 the Tropics ever start northward, as in their winter quarters the varia- 

 tion in the length of day from winter to summer is imperceptible?" 

 Like all the other theories advanced, this also, as at present under- 

 stood, is subject to unanswered criticism. 



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