Graduate Thesis Or Dissertation
 

Food and habitat partitioning in two groups of coexisting Accipiter

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  • Sharp-shinned Hawks (Accipiter striatus), and Cooper's Hawks (A. coo erii) in the conifer forests in northwestern Oregon and A. striatus, A. cooperii, and Goshawks (A. gentilis) in the conifer forests of eastern Oregon were syntopic during the nesting season. In this study density of nests, nest success, and utilization of food and habitat resources by each Accipiter in both northwestern and eastern Oregon were determined for 1969 through. 1974. Abundance of birds and small, diurnal mammals in the forests of eastern Oregon and the abundance of birds in the forests of northwestern Oregon were estimated in faunal surveys of a variety of conifer forest types. In addition, the occurrence of small mammals in eastern Oregon and their relative abundances were qualitatively estimated. The physiography and vegetative structure of Accipiter nest sites were quantified in both study areas, and differences in habitat variables among the sites of each species were examined by multivariate analysis of variance and discriminant analysis. These analyses showed that, while nest sites of each Accipiter in both areas had similar physiographic features, they differed in the structure of the vegetation. These differences were associated with the successional stage of the forest stands selected by each species: A. striatus nested in dense, 40-60-year-old, even-aged stands of conifers, A. cooperii in 50-80-year-old, dense conifer stands with slightly larger, more widely spaced trees, and A. gentilis in dense, conifer stands with large overstory trees and a dense understory of regenerating conifers. The following differences in the prey resources of the two study areas were noted; eastern Oregon had (1) nearly one half the species and density of birds, (2) a size-frequency distribution shifted somewhat toward larger birds, and (3) more species and a greater density of mammals than northwestern Oregon. Mean size of prey (12.8 g) captured by A. striatus in northwestern Oregon was significantly smaller than prey (28.4 g) captured by the same species in eastern Oregon. The diet of A. striatus in both areas was composed almost entirely of birds (<5% mammals). Mean size of prey of A. cooperii in both areas was nearly the same (134.7 g and 136.3 g) and each was significantly larger than mean sizes of prey of A. striatus. However, prey size and taxonomic composition of diets of A. cooperii varied; 74% birds (x = 79.2 g) and 25% mammals (x = 296.4 g) in northwestern Oregon and 47% birds (x = 123.7 g) and 53% mammals (x = 147.5 g) in eastern Oregon. Mean size of prey (306.6 g) of A. gentilis in eastern Oregon was significantly larger than mean size of prey of A. cooperii and consisted of 53% birds (3T = 195.5 g) and 45% mammals (x = 445.2 g). An analysis of the utilization of prey sizes, prey taxa, and foraging height zones demonstrated that partitioning of food in both areas occurred primarily in the prey size dimension. However, A. cooperii and A. gentilis, which broadly overlapped in the size dimension, showed increased partitioning of prey taxa. In eastern Oregon, use of foraging zones by A. cooperii and A. gentilis was nearly identical; both species captured prey primarily from the lower zones (ground-shrub and shrub-canopy). There was, however, little overlap on this dimension between A. striatus and its congeners in either study area as A. striatus almost entirely limited its foraging to the upper canopy. Although it was not determined if food resources were limiting, a review of the feeding ecology of Accipiter in Oregon, in view of some predictions of competition theory, supported the hypothesis that the different body sizes of coexisting members of this genus evolved in response to competition for food resources during the breeding season.
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