The red-tailed hawk on Sauvie Island, Oregon

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