Graduate Thesis Or Dissertation

Effects of oil and natural gas development on territory occupancy of ferruginous hawks and golden eagles in Wyoming, USA

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  • Energy development is expanding rapidly across the western US. Negative effects have been documented for some wildlife, but consequences of development are unclear for other taxa, including raptors. We had the opportunity to examine effects of oil and natural gas development on two raptor species of conservation concern, ferruginous hawks (Buteo regalis) and golden eagles (Aquila chrysaetos), in sagebrush steppe and prairie habitats of Wyoming. We surveyed nest sites of these species using fixed-wing aircraft during 2010–2011, and monitored occupancy of the resulting sample of historically active breeding territories during 2011–2013 for ferruginous hawks, and 2012–2013 for golden eagles. We used single-season occupancy models to evaluate post-construction effects of oil and natural gas development in the context of other factors predicted to influence use of territories by these species, including prey abundance, nest site characteristics, and vegetation. An additional objective was to demonstrate a monitoring protocol for raptors in Wyoming that used probabilistic sampling and accounted for imperfect detection. In support of our predictions, probability of territory occupancy by ferruginous hawks had a strong positive relationship to abundance of ground squirrels (Urocitellus spp.), a strong negative relationship to vegetative cover of sagebrush (Artemisia spp.), and was slightly higher for artificial nest platforms compared to other substrates; and territory occupancy for golden eagles had a strong positive relationship to nest height. Contrary to our predictions, density of oil and natural gas infrastructure was not strongly related to occupancy for either species, and prey abundance was not related to occupancy for golden eagles. The only anthropogenic factor that influenced occupancy for either species was density of improved roads not associated with oil and natural gas fields, which had a weak positive correlation with occupancy for ferruginous hawks, contrary to our predictions. Annual occupancy probability did not vary significantly for either species during our study, but environmental factors associated with occupancy and the strength of relationships varied among years for both species, suggesting occupancy was influenced by additional factors not included in our analysis (e.g. weather, regional dynamics). Detection probability for both species was <1, and strongly influenced by nest substrates. For ferruginous hawks, detection probability varied significantly between years, and was positively associated with nest height. For golden eagles, detection probability was significantly higher in territories with nests on trees, shrubs, and anthropogenic structures, compared to those on cliffs and rock outcrops, with a weak negative trend in detection rates across survey occasions during one year. Our results suggest ferruginous hawks and golden eagles used breeding territories that contained active oil and gas roads and well pads, and density of infrastructure in these territories did not affect their probability of use. However, we advise that limitations of our approach (i.e. post-construction, short-term, observational study) make our results most relevant as a baseline for ongoing monitoring of these species. We suggest protection efforts should be focused on ferruginous hawk territories with abundant ground squirrels and low natural cover of sagebrush, and golden eagle territories with higher nest sites. We recommend conserving populations and habitats of burrowing mammals, mitigating loss of nests using artificial platforms, and long-term monitoring of ferruginous hawks and golden eagles using robust methods that account for imperfect detection.
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