Forest Structural Characteristics of Accipiter Nesting Habitat: Is There an Allometric Relationship?

Abstract

In montane forests of the western United States, a general correlation of Accipiter body size and scaling of the vegetation component of nesting sites and nest trees used by sympatric Accipiter species has been reported. We evaluated this pattern with vegetation data collected at Northern Goshawk (Accipiter gentilis), Cooper's Hawk (A. cooperii), and Sharp-shinned Hawk (A. striatus) nest sites in the Jemez Mountains and Pajarito Plateau of north-central New Mexico. We selected habitat variables at the nest tree and nest site scale that would allow us to evaluate the prediction that accipiters use nesting habitat in which their body size is positively correlated with tree size and tree spacing, and inversely correlated with tree density, basal area, and percent canopy closure. At the nest-site level, density of larger diameter trees should be positively correlated with body size, and density of smaller diameter trees should be inversely correlated with body size. Our results suggest that nest tree height and diameter support body size predictions about nesting habitat for accipiter hawks. None of the nest-site parameters measured in this study supported the body size predictions due to a large amount of intra-specific variation. As a result ofthis variation, it was difficult to differentiate between Cooper's Hawk and Northern Goshawk nest sites for most site variables. These results suggest there is a correlation between accipiter size and nest tree size, but that a correlation between nest site structural size and accipiter body size may not be a widespread phenomenon for all vegetation variables for all three species. Many commonly measured forest stand structural characteristics such as basal area and total tree densities may not be adequate for predicting suitable accipiter nesting habitat in all areas, particularly in the absence of comparisons with available habitat.