After the 28th day, only 18 food items, or slightly less than 20 per cent of the total number, were brought to the nest. These last 18 food items brought after the owls were 4 weeks old were no larger or bulkier than those brought in the previous 20 days. The beginning of this period of reduced amount of food corresponds to the beginning of the second phase of growth characterized by marked fluctuations in weight. Fox squirrels (_Sciurus niger_) are abundant on the University campus, yet there were no remains of this mammal at the nest. This may be explained by the fact that fox squirrels are principally diurnal and Great Horned Owls feed principally at night. Yet in early February, 1946, when the owls were preparing the nest, they frequently flew on and off the nest in the early twilight of evening while one or two fox squirrels fed in the periphery of trees not more than 25 feet away. Yet the owls flew off to the west and left this source of food unmolested. Whether both owls regularly attended the young we do not know, for the adults were not distinctively marked. On March 17, 1945, when weighing the young, one parent bird started to return to the nest but was frightened away by the observer who at the same time noted the other parent perched in an adjacent tree. This was the first time two adults were seen at the same time near the nest. In 1946, two adult owls (presumably both were parents) were within sight at one time when the young owl first sailed forth and landed in a wooded area some 100 yards away. SUMMARY Great Horned Owls (_Bubo virginianus virginianus_) have employed as nest sites the protruding shelves of the stone wall of the Museum of Natural History at the University of Kansas for several years. In 1945, daily observations were made on one such nest and its three young, and in 1946 irregular observations were made on another such nest and the one young owl. The incubation time for the three owls, hatched in 1945, was 35 days for two of the young and 34 days for the third; for the one owl hatched in 1946, the incubation time was at least 33 days. Two owls were consistently smaller; when these smaller two left the nest they were, respectively, 21 and 17 per cent lighter than the other two. The smaller two were judged to be males because adult males in Kansas average smaller by 21 per cent than adult females. Growth of the nestling young is divisible into (1) a period of rapid increase in weight during the first 25 to 28 days, and (2) a subsequent period marked by gains and losses in weight. The fluctuations in this latter period are correlated with a reduction in food brought to the nest by the parent birds and with the development of habits of flight. This second period may be considered to be a period of "weaning." By the forty-fifth day, the young owls are able to fly short distances and thus are able to leave the site of the nest permanently. At this time they are about three-fourths grown.