NESTING PAIR DENSITY AND ABUNDANCE OF FERRUGINOUS HAWKS (BUTEO REGALIS) AND GOLDEN EAGLES (AQUILA CHRYSAETOS) FROM AERIAL SURVEYS IN WYOMING

摘要

Raptors that inhabit sagebrush steppe and grassland ecosystems in the western United States may be threatened by continued loss and modification of their habitat due to energy development, conversion to agriculture, and human encroachment. Actions to protect these species are hampered by a lack of reliable data on such basic information as population size and density. We estimated density and abundance of nesting pairs of Ferruginous Hawks (Buteo regalis) and Golden Eagles (Aquila chrysaetos) in sagebrush steppe and grassland regions of Wyoming, based on aerial line transect surveys of randomly selected townships. In 2010 and 2011, we surveyed 99 townships and located 62 occupied Ferruginous Hawk nests and 36 occupied Golden Eagle nests. We used distance sampling to estimate a nesting pair density of 94.7 km(2) per pair (95% CI: 69.9-139.8 km(2)) for Ferruginous Hawks, and 165.9 km(2) per pair (95% CI: 126.8-230.8 km(2)) for Golden Eagles. Our estimates were similar to or lower than those from other studies in similar locations in previous years; thus, we recommend continued monitoring to determine trends in nesting pair density over time. Additionally, we performed double-observer surveys on a subset of transects with a helicopter as the second observation aircraft. We estimated probability of detecting occupied nests from fixed-wing plane versus helicopter, as well as time and expense of each survey mode. Although observers surveying from helicopters were 1.19 and 1.12 times more likely to detect Ferruginous Hawk and Golden Eagle occupied nests, respectively, the helicopter survey was 4.55 times costlier due to longer flight time and the higher hourly costs. Thus, when systematically surveying large areas, we found cost and time of the helicopter surveys outweighed the increase in nest detection.