Ninety-one individuals of 16 species of birds and mammals made up the food items brought to the nest in 1945. Two factors seem to be concerned in the acquisition of prey: (1) its availability and (2) appropriate size of the prey. LITERATURE CITED BAUMGARTNER, F. M. 1938. Courtship and nesting of the Great Horned Owls. Wilson Bull., 50:274-285. BENT, A. C. 1938. Life histories of North American birds of prey (Part 2), Orders Falconiformes and Strigiformes. U. S. Nat. Mus., Bull. 170, viii + 482 pp. FITCH, H. S., SWENSON, F., and TILLOTSON, D. F. 1946. Behavior and food habits of the Red-tailed Hawk. Condor, 48:205-237. HUGGINS, S. E. 1940. Relative growth in the House Wren. Growth, 4:225-236. RIDDLE, O., CHARLES, D. R., and CAUTHEN, G. E. 1932. Relative growth rates in large and small races of pigeons. Proc. Soc. Exp. Biol. and Med., 29:1216-1220. STONER, D. 1935. Temperature and growth studies on the Barn Swallow. Auk, 52:400-407. 1945. Temperature and growth studies of the Northern Cliff Swallow. Auk, 62:207-216. SUMNER, E. L., JR. 1929a. Notes on the growth and behavior of young Golden Eagles. Auk, 46:161-169. 1929b. Comparative studies in the growth of young raptores. Condor, 31:85-111. 1933. The growth of some young raptorial birds. Univ. California Publ. Zoöl., 40:277-307. 1934. The behavior of some young raptorial birds. Univ. California Publ. Zoöl., 40:331-361. [Illustration: FIG. 3. Young Great Horned Owls in nest. Two owls are 7, 12, 18, 32, and 36 days of age, respectively; the third owl is about 2 days younger in each instance.] [Illustration: FIG. 4. Young Great Horned Owl hatched in 1946. The two lower pictures show the developing facial mask. Photographs by João Moojen.] [Illustration: FIG. 5. Young Great Horned Owl hatched in 1946. Upper row: Ventral views showing scar of yolk sac and ventral side of wing. Middle row: Ventral (left) and dorsal view of wing at 21 days. Bottom row: Ventral (left) and dorsal view of wing at 47 days. Photographs by João Moojen.] 21-6958