Red-Tailed Hawk and Horned Owl Populations in Wisconsin

In the winter of 1953 we began a three-year investigation of interactions between populations of the Red-tailed Hawk (Buteo jamaicensis) and the Horned Owl (Bubo virginianus) on 9.5 square miles of open farm land in southern Green County, Wisconsin.The study area included all of Sylvester and Jefferson townships and parts of Clarno and Monroe townships.Additional observations were made on land within one-quarter mile of the area since birds &sting there fed partly in the study area.In the three-year period 90 nests of Red-tails and 48 nests of Horned Owls were examined.The original vegetation of the area, as determined from surveyors' records by Dr. J. T. Curtis, Department of Botany, University of Wisconsin, consisted of a tongue of maple (Acer saccharum)-basswood (Tiliu americana) forest bordered on the east and northwest by a belt of oak-basswood (Quercus-Tilia) forest which continued across the north-central portion of the area.The southeast part and most of the northern edge of the a.rea were covered primarily by tall grass prairie with small islands and extensions of the forest.Extensive agriculture has greatly altered this original vegetation so that only small woodlots remain (fig.1).Maple-basswood has replaced the oak-basswood chiefly on the north slopes, but oak dominates the drier slopes.The prairie areas are still essentially treeless with only a few scattered fence-row willows (Sal&), American elms (Ulmus americana), and black cherries (Prunus serotina) (fig.2).The principal farm crops are corn, hay, and oats.Much of the land is in pasture.The other avian raptors breeding on the area were Screech Owls (O&s asio), a few Sparrow Hawks (Falco sparverius), and Cooper Hawks (Accipiter cooperi).Broadwinged Hawks (Buteo pZutypterus) , Sharp-shinned Hawks (Accipiter striatus) , and Marsh Hawks (Circus cyaneus) visited the area during migration, and Short-eared Owls (Ash flammeus) were uncommon in winter.Rough-legged Hawks (Buteo Zagopus), sometimes as common as Red-tails between mid-November and mid-April, were the only other large raptors on the area in any numbers.The word "competition" has been used to refer to a confusing variety of relationships between individuals, but recently Andrewartha and Birch (1954) have given good reasons for restricting the use of that term.Therefore, in this paper we have adopted the following terminology: Interaction, any relationship among individuals of the same or differing species; competition, the demand of individuals upon any common resource which is in short supply; and interference, any detrimental interaction not falling into the class of competition as just defined.