It was assumed that the migratory habit of most birds was inherent, and that food, cold, and similar factors could not account for it. The regularity of dates of departure suggested that some environmental factor such as day-lengths might be involved. An internal stimulus seemed probable. Birds migrate northward as the gonads enlarge and southward as they decrease. It was assumed that day-lengths control the physiological stimulus to migrate. Of 83 crows (Corvus b. brachyrhynchos) housed in outdoor aviaries in Alberta, one group of 69 was subjected, by means of electric lights, to gradually lengthening days in Sept., Oct., and Nov. A group of 14 controls had no artificial lighting. Both groups were released on Nov. 9, after developing interstitial tissue had been indicated in the experimentals. Of the 14 controls, 6 remained; some of the others went SE. (up to 200 miles), but none were reported from the N. or NW. Of the 69 experimentals, 54 ultimately left, and 28 of these were reported killed[long dash]12 locally, 8 S. and SE., and 8 N. and NW.