A study was undertaken to determine the abundance, distribution, perching heights, diets and territory sizes of Red-Tailed Hawks on Sauvie Island, Oregon. Sauvie Island supported a large wintering population of non-territorial Red-Tailed Hawks. The establishment of territories began around 10 January. Average perch height was found to be 14.2 m. Average perch height increased from fall to spring, corresponding-to the onset of the breeding season and to the onset of egg-laying and incubation. Voles (Microtus spp.) were the principal prey year-round, though waterfowl were more important in terms of biomass in the winter and early spring. Territory sizes on the Island ranged from .31 - 3.73 km squared The abundance of voles was assessed in different habitats within five Red-Tailed Hawk territories. Territory size was shown to be inversely correlated to the population density of voles. The implications of this finding for the type of territoriality exhibited by Red-Tailed Hawks is discussed, as are some proximate factors affecting habitat selection.

This study is an analysis of the various factors affecting the population of raptorial birds on Sauvie Island, Oregon. A census of diurnal and nocturnal raptors was carried out along with an analysis of food habits. Once the major prey species were determined they were censused, and the effects of the land management practices on their numbers was investigated. In addition, 100 randomly selected nest boxes were sampled so as to determine the extent to which raptors were making use of them. Red-tailed hawk and Great Horned Owl nests were located It was found that four diurnal raptors and three nocturnal raptors were present in sizeable numbers on Sauvie during the winter and spring of 1972. In addition here were less common sightings of four diurnal and one nocturnal raptor. The most common food item of raptors in general was the vole, Microtus townsendi, which was later found to be present in extremely high numbers. The common practice of planting fields of grains and the intentional flooding of them (for waterfowl use) was found to be a major factor in the numbers and vulnerability of Microtus townsendi. Ducks were a common food item for all raptorial species during and immediately after duck hunting season, but not at any other time of year which seemed to indicate that the raptors were feeding on carrion or wounded ducks. It was found that Barn Owls (Tyto alba), Screech Owls (Otis asio) and Kestrel (Falco sparvarius) made use of the nest boxes on Sauvie Island. The overlapping of food habits of Sauvie Island raptors was discussed and an attempt was made to reconcile the apparent contradiction to Gause’s Rule.