the Great Horned Owls under observation, and in some other species as indicated by published growth curves, the initial period of slow gain is lacking. The period of a decelerated rate of growth in the young Great Horned Owls is correlated with the occasional lack of food. The parent birds, during this latter period, remain off the nest more of the time during the day, and their failure to provide the young with food may represent an attempt to force the young to become proficient in flight or to force them away from the nest site, which amounts to the same thing. When only slightly more than a month old, the young began to test their wings, springing into the air, and, in general, becoming more active and alert. Sumner (1929b:110) has suggested some other possible reasons for the period of decelerated rate of growth. Although there was a daily increase in weight in the early stages of growth, there was a decided fluctuation in any twenty-four hour period. On any given day, the young always were heavier in the morning than in the afternoon (see figure 2); presumably they were gorged with food early in the morning. [Illustration: FIG. 2. Morning and afternoon weights of two Great Horned Owls. Note that in the morning the owls weigh more than in the afternoon.] When the young left the nest, they were approximately three-fourths grown. When owl I on the 44th day and owl II on the 45th day left the nest, they weighed 1,120 and 1,139 grams, respectively, or 73 and 74 per cent, respectively, of the average weight of 25 females (1,530.9 grams). Owl III weighed 943.3 grams on the 43rd day and owl IV weighed 971.4 grams on the 49th day, or 78 and 80 per cent, respectively, of the average weight of 17 mature males (1,207.7 grams). Owl I left the nest 18 hours before owl II did. Owl III attempted to leave when 43 days old, but for it coördinated flight was impossible and the bird landed on the lawn after a 150-foot glide. When attempting to take owl IV from the nest on the 49th day, it sprang into the air and by gliding, aided by an occasional flap, sailed more than 300 feet before alighting on the ground. After we returned the owl to the nest, it immediately sailed forth for even a longer distance. When attempt was made to pick up owls III and IV after they had flown down to the ground, they rolled over on their backs and used both claws and beaks defensively. Such a reaction never was noted at the nest; there our hands were inspected, and sometimes bitten by the owls as possible sources of food, but the claws were rarely used offensively or defensively.