Resource partitioning and reproductive success of three species of hawks (Buteo spp.) in an Oregon prairie

Graduation date: 1982In a native prairie of Oregon (363 km²) red-tailed (Buteo jamaicensis), ferruginous (Buteo regalis), and Swainson's hawks (Buteo swainsoni) coexist in one of the highest nesting Buteo densities in North America. In 1979 and 1980, the distribution, density, and productivity of the three species was studied in relation to availability of food and nesting habitat. The degree of ecological segregation among the three species was also investigated. Red-tailed and ferruginous hawks showed considerable overlap in the timing of their nesting cycles; the Swainson's hawk nested about one month later in the spring. The major food item of all three species was the Belding ground squirrel (Spennophilus beldingi). The total dietary overlap among the three hawk species was high: in 1979 and 1980, red-tailed and ferruginous hawks overlapped 90 percent and 80 percent, red-tailed and Swainson's hawks 88 percent and 83 percent, and ferruginous and Swainson's hawks 83 percent and 95 percent. Although timing and use of food resources exhibited considerable overlap, habitat was partitioned. Nine major habitat types occurred within the study area. Red-tailed hawks had almost exclusive use of pine groves and cultivated areas with cottonwood trees. Ferruginous hawks had almost exclusive use of south exposure sites where large rocky outcrops occurred. Use of other habitat types overlapped to some degree. The total habitat overlap between the red-tailed and ferruginous hawk in 1979 and 1980 was 33 percent and 37 percent. The Swainson's hawk exhibited slightly greater overlaps in 1979 and 1980 of 54 percent and 52 percent with red-tailed hawks and 47 percent and 41 percent with ferruginous hawks. Habitat overlap of the Swainson's hawk tended to be minimized by its selection of nest trees whose configuration differed from red-tailed and ferruginous hawks. Space was also partitioned by the maintenance of intraspecific and, to a lesser degree, interspecific nesting territories. Nest success of red-tailed and ferruginous hawks significantly declined when nesting pairs were visible to one another. This suggests that competition for space was a mechanism affecting distribution, density, and nest success. Based on the findings from this study, Buteo management could involve: (1) range management for prey species, which in this case is in accord with good management for cattle, (2) range management for nesting habitat, and (3) maintenance of properly spaced nesting sites