Nesting Habitat of Coexisting Accipiter in Oregon

Abstract

The distribution of nesting pairs of 3 species of Accipiter and the habitat they used for nesting in the conifer forests of Oregon were determined during spring and summer 1969-74. Each species nested in all forested mountain ranges surveyed except the Coast Range in northwestern Oregon, where goshawks (Accipiter gentilis) were absent. A multivariate analysis of variance of the vegetation at Accipiter nest sites showed that each coexisting species used habitats with different structures and that the structural differences among the nest sites were associated with the ages of the forest stands used. Nest sites were characterized as dense, 40-60-year-old even-aged conifer stands for sharp-shinned hawks (A. striatus); dense, 50-80-year-old conifer stands with somewhat larger, more widely spaced trees for Cooper's hawks (A. cooperii); and dense, mature conifer stands with varying densities of mature, overstory trees and, dependent upon the density of the overstory, dense to open understories of smaller, shade-tolerant conifers for goshawks. Physiographies of the nest sites of the hawks were similar; each species tended to nest in stands that occurred on gentle slopes with northwest to northeast exposures. J. WILDL. MANAGE. 46(1):124-138 Forest managers have recently noted decreases in some forest bird populations following timber harvest. A desire to minimize the effects of timber harvest on bird populations has placed a premium on knowledge of the specific habitat needs of the affected species. It is necessary, for example, to identify the structural characteristics of the habitat that elicit selection of a site by a species (Hilden 1965), to identify the vegetative characteristics that produce the physical environment required for successful nesting (Horvarth 1964, Ricklefs and Hainsworth 1969), and to identify the types, amounts, and spatial distribution of the vegetation that provides the required food types of those species sensitive to timber harvest. Theoretically, the structural aspects of the vegetation that elicit selection (e.g., terrain, vegetation type, and density) should indicate, or be correlated with, habitats that contain requisite resources, protection from predators, or environmental factors to which a species is adapted. Within Oregon, sharp-shinned hawks, Cooper's hawks, and goshawks are syntopic forest raptors that feed primarily on birds and mammals (Reynolds and Wight 1978, Reynolds 1979). During our field studies we noted that each species of Accipiter in Oregon nested in habitats with specific structure, and that this specificity made them susceptible to changes in forest stands brought about by timber management. Thus, the objectives of this study were to (1) describe the vegetation and topography of the nesting habitats in 2 geographic regions of Oregon; (2) examine differences in the vegetative structure of the habitats used for nesting by sharp-shinned hawks, Cooper's hawks, and goshawks in both regions; and (3) examine how the characteristics of the vegetation of the nesting habitat might be I This study was conducted under the auspices of the Oregon Cooperative Wildlife Research Unit, Oregon State University, Oregon Department of Fish and Wildlife, U.S. Fish and Wildlife Service, and The Wildlife Management Institute cooperating. This is Oregon Agriculture Experiment Station Technical Paper 5781. 2 Present address: Rocky Mountain Forest and Range Experiment Station, 240 West Prospect, Fort Collins, CO 80526. 3 Deceased. 124 J. Wildl. Manage. 46(1):1982 This content downloaded from 157.55.39.111 on Tue, 02 Aug 2016 05:49:47 UTC All use subject to http://about.jstor.org/terms ACCIPITER NESTING HABITAT IN OREGON' Reynolds et al. 125 related to the physiological and behavioral adaptations associated with Accipiter nesting ecology. This study was supported by the USDA Forest Service, Pacific Northwest Forest and Range Experiment Station; U.S. Fish and Wildlife Service; and the Oregon Department of Fish and Wildlife. We thank E. Forsman, G. Cornett, J. Tabor, B. Heckel, L. Hunt, and W. Pike for field assistance, and R. Anthony, E. Forsman, T. Gavin, and R. King for suggestions and manuscript review.