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Leary, Alan W. (Al) – Home ranges, core use areas, and dietary habits of Ferruginous Hawks in southcentral Washington

Ferrugineous hawk on branchDuring the 1994 and 1995 breeding seasons, I used radio-telemetry to track movements of seven adult male ferruginous hawks (Buteo regales) in south-central Washington. Locations were taken randomly during all daylight hours throughout the breeding season with a minimum of one hour between successive locations. Home range size was estimated using minimum convex polygon, harmonic mean, and kernel-based methods. Minimum convex polygon home ranges were significantly (10X) larger than those previously reported for ferruginous hawks. Home ranges varied significantly among males (mean = 78.6 km 2, range = 8.9 – 136.4 km2). There was no relationship between home range size and brood size, but there was a significant relationship between home range size and the distance from the nest site to the nearest irrigated agricultural field where some males hunted. Adaptive kernel-based estimates identified core areas of high use within home ranges. Kernel-based estimates showed two distinct core areas, one around the nest and a second in the agricultural fields where males hunted.

Dietary data were collected by direct observations at eight ferruginous hawk (Buteo regales) nests in southcentral Washington during the 1994 and 1995 breeding seasons. Observations were conducted at least once each week at all nests during one of three observation periods which covered all daylight hours. A total of 203 prey deliveries were observed. Composition of ferruginous hawk diets varied between years, but mammals comprised 89.8% and 78.6% of the diet in 1994 and 1995, respectively. Of the prey items observed, 52% were medium-sized mammals such as northern pocket gophers (Thomomys talpoides) and 66% were brought to nests during the midday observation period (1000-1 559H). Highest delivery rates (deliveries / hr of observation) occurred between 1400 – 1559H. Pre-fledging delivery rates of food were significantly higher than post-fledging rates, but pre-fledging rates varied significantly among nests. There was a significant relationship between both delivery rate and brood size and delivery rate and the size of prey items delivered.

Thesis Abstract

Where Are They Now?