THE MIGRATION OF NORTH AMERICAN BIRDS a 
dant food in the South in spring and press toward northern points 
when the food supplies there are almost entirely lacking and when 
severe cold and storms are likely to play havoc with the advance 
migrants. The regularity of arrival and departure is one of the most 
impressive features of migration, and since birds travel in almost 
strict accordance with the calendar, the proponents of the theory ask: 
What phenomenon to which we may attribute the stimulating im- 
pulse occurs with such precise regularity as the constantly increasing 
hght in spring? 
Experimental work has abundantly demonstrated the effect of 
increased light upon the growth, flowering, and fruiting of plants. 
Similarly, experiments with the common junco, or-snowbird (Junco 
hyemalis), reported by Rowan (42, p. 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 Novem- 
ber until early in January, the juncos were subjected to ever-increasing 
light, supplied in the aviaries by electric bulbs. As regards illumina- 
tion, they were thus artificially provided with conditions approxi- 
mating 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 
hghting over a period of little more than 1 month, the organs returned 
to their normal winter condition. 
After a consideration of all evidence, including the fact that no 
ultraviolet 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 
variation 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. 
WHEN BIRDS MIGRATE 
It is known that at any given point many species leave in fall and 
return In spring. Since bird banding has had such wide application 
as a method of study, it is known also that in some species one of the 
