GOLDEN EAGLE DIET BREADTH AND REPRODUCTION IN RELATION TO FLUCTUATIONS IN PRIMARY PREY ABUNDANCE IN WYOMING'S BIGHORN BASIN

Golden Eagles (Aquila chrysaetos) are facing rapid environmental changes that may reduce the abundance and distribution of their prey in the western United States. Mitigation of negative effects depends in part on understanding Golden Eagle diet and the relationship between prey abundance and eagle reproduction. We documented reproduction and diet for Golden Eagles nesting in Wyoming's Bighorn Basin 2009-2015 and examined relationships between primary prey abundance and Golden Eagle diet breadth and reproductive rate. Annual Golden Eagle reproductive rate averaged 0.73 (range = 0.38-1.32) fledglings per occupied nesting territory. Cottontails (Sylvilagus spp.) dominated the diet in each year of the study, although cottontail abundance fluctuated markedly from year to year. The annual occurrence of cottontails in prey remains collected from nests ranged from 60.1% to 90.9% (frequency) and from 46% to 96% (biomass). Annual Golden Eagle diet breadth (Levins index) averaged 1.91 (frequency) and 1.98 (biomass). Both measures of annual diet breadth were negatively related to cottontail abundance. Annual eagle reproductive rate increased significantly as cottontail abundance increased (r(2) = 0.78, F-1,F-5 = 17.35, P = 0.009). Our results suggest that the abundance of cottontails was the critical factor influencing Golden Eagle reproduction in a given year during our study. To mitigate negative effects of environmental changes on the Golden Eagle population in the Bighorn Basin, we suggest maintaining or improving habitat conditions that support robust cottontail populations and improving conditions for potential alternative prey species where ecologically feasible and socially acceptable.