Partitioning of Food and Niche Characteristics of Coexisting Accipiter During Breeding

Abstract

AbstractThe abundance of resources and their use by Accipiter in two areas within Oregon from 1969 through 1974 are presented. In the coastal mountains of northwestern Oregon only Sharp-shinned Hawks (Accipiter striatus) and Cooper's Hawks (A. cooperii) nested. In the remainder of the state, A. striatus, A. cooperii, and Northern Goshawks (A. gentilis) were syntopic during the nesting season. The number of species and the total density of birds in eastern Oregon were about one-half those in northwestern Oregon. Mean prey size of A. striatus in northwestern Oregon (12.8 g) was significantly smaller than that of the same species in eastern Oregon (28.4 g). Accipiter striatus diets in both areas were composed almost entirely of birds (<5% mammals). Mean prey size of A. cooperii in both areas was nearly the same (134.7 g vs. 136.3 g) and was significantly larger than mean prey size of A. striatus. The frequency of occurrence and the size of prey taxa in A. cooperii diets, however, varied between areas: 74% birds (x̄ = 79.2 g) and 25% mammals (x̄ = 296.4 g) in northwestern Oregon versus 47% birds (x̄ = 123.7 g) and 53% mammals (x̄ = 147.5 g) in eastern Oregon. Mean prey size of A. gentilis in eastern Oregon (306.6 g) was significantly larger than that of A. cooperii and consisted of 55% birds (x̄ = 195.5 g) and 45% mammals (x̄ = 445.2 g). An analysis of prey sizes, prey taxa, and foraging heights indicated that, in general, food was partitioned in both areas primarily according to prey size. In eastern Oregon, however, where A. cooperii and A. gentilis overlapped broadly in prey size, these species tended to take different prey taxa. Both A. cooperii and A. gentilis foraged primarily in the lower zones (ground-shrub and shrub-canopy), whereas A. striatus foraged in the upper canopy.