Along with other predatory species, the great horned owl has been studied in north-central United States, particularly in Iowa and southern Wisconsin localities in which ecological research upon certain prey types has been carried on contemporaneously. After investigation by field and laboratory methods involving experimentation with captive horned owls and observation in nature, it became obvious that the mass data on feeding trends required by the program could be best obtained through pellet studies supplemented by whatever additional techniques would yield information.

The horned owl food habits data upon which this bulletin is partly based were gathered largely between 1930 and 1935 and were the product of experience with 84 horned owl nests, examination of 4,815 pellets and 23 food-containing stomachs and records of direct predation. The prey types studied were chiefly upland game birds, waterfowl and fur-bearers, though many other forms were observed incidentally. The general procedure was to continue, so far as feasible, work on predation and population year after year on specific areas and to correlate the data from both; in this way, not only have some of the reasons for pronounced changes in food habits of predators become evident, but a superior background for evaluating effects of predation upon population levels of prey species has also resulted.