- 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
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.