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 themselves--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 understood, 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 parent birds (rarely both) frequently returns and nests in the same tree, bush, or box that held its nest in the previous season (fig. 1). One ordinarily thinks of the world of birds as quiescent during two seasons each year, at nesting time and in winter. For individual species this is obviously the case, but when the entire avifauna of the continent is considered it is found that there are at almost all periods some latitudinal movements. [Illustration: B12M Figure 1.--The bluebird may return regularly year after year to nest in the same hole or box that was occupied in previous seasons.] MOVEMENTS OF SPECIES AND GROUPS