Miller, Robert

Breeding ecology of the Northern Goshawk (Accipiter gentilis) within a unique prey and forest landscape of the western United States. 2013

Abstract

The high-elevation mixed aspen-lodgepole pine and shrub-steppe landscapes of the northern Great Basin in the western United States provide unique challenges for a forest predator such as the Northern Goshawk (Accipiter gentilis). The forests naturally lack common goshawk prey species such as tree squirrels of the genera Sciurus and Tamiasciurus, and other preferred prey species such as corvids are uncommon. The forest structure is highly fragmented and makes up only about 20% of the shrub-steppe dominated landscape limiting cover, nesting locations, and other forest aspects preferred by goshawks. I studied many aspects of the breeding ecology of goshawks in the area including the breeding season diet, nest stand attributes, and the influence of prey abundance, nest stand structure, and territory structure on nest occupancy and productivity.

A critical element of diet analysis is species adaptability to alternative prey sources. The breeding season diet of goshawks includes both mammalian and avian species, varies geographically, and is often dependent upon tree squirrels of the genera Sciurus and Tamiasciurus. I quantified the diet of goshawks using nest cameras and surveyed abundance of prey using line transects. I found that goshawks consumed roughly 18.5% birds and 78.7% mammals by biomass, dominated specifically by the Belding’s ground squirrel (Spermophilus beldingi; 74.8% of total biomass consumed). This diet was low in diversity with high overlap among nests indicating a strong local dependence on this specific diet. Higher avian prey abundance early in the nesting season seemed to drive an increased consumption of mammalian prey later in the nesting season. The availability of mammalian prey during the egg-laying phase of the breeding season was limited by hibernation. In turn, consumption of avian prey early in the nesting cycle likely influenced brood sizes. Larger brood sizes increased the demand for prey biomass later in the nesting season, resulting in a switch to heavier mammalian prey. This study provides new insight into the adaptability of the goshawk diet and the complex indirect relationships often required for breeding success.

I developed a habitat suitability model for predicting nest locations of breeding goshawks. I used elevation, slope, aspect, ruggedness, distance-to-water, canopy cover, and individual bands of Landsat imagery as predictors for known nest locations with logistic regression. I found goshawks prefer to nest in gently-sloping, east-facing, non-rugged areas of dense aspen and lodgepole pine forests with low reflectance in green (0.53 – 0.61 µm) wavelengths during the breeding season. I used the model results to classify my 43,169 hectare study area into nesting suitability categories: well suited (8.8%), marginally suited (5.1%), and poorly suited (86.1%). I evaluated my model’s performance by comparing the modeled results to a set of GPS locations of known nests (n = 15) that were not used to develop the model. Observed nest locations matched model results 93.3% of the time for well suited habitat and fell within poorly suited areas only 6.7% of the time. My method improves on goshawk nesting models developed previously by others and may be applicable for surveying goshawks in adjacent mountain ranges across the northern Great Basin.

I evaluated the influence of prey abundance and forest structure on the occupancy and productivity of the goshawk. I combined multi-species prey surveys based on distance sampling and habitat structure at the nest stand and territory level to predict goshawk nest occupancy and productivity. I accounted for uncertainty in prey estimates and goshawk detection by with hierarchical bayesian analysis. I found that avian prey abundance, but not mammalian prey abundance, influenced territory occupancy in 2011, but in 2012 no effect of prey abundance was detected. Nest stand and territory habitat structure failed to show an effect on occupancy in either year. I found no effect of prey abundance or habitat structure on within year productivity, but did note an increase in avian abundance among years that coincided with an increase in productivity, although the later was not significant. Avian abundance may be more influential on nest survival than nest occupancy, especially when mammalian prey is delayed by late spring conditions. Ground squirrels are the dominant prey of goshawks in the area, and once they emerge they appear to be sufficiently abundant for goshawk breeding success, even in territories with low relative squirrel abundance. Lastly, I suggest that goshawks show little sensitivity to the relative structural quality or prey abundance of a territory or nest stand, at least among previously acceptable territories.

Thesis

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