Ferruginous Hawk Populations and Habitat Use in North Dakota

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Abstract
Ferruginous hawks (B. regalis) were studied in central North Dakota on a 1259-km2 intensive study block and on a surrounding study area (16,519 km2) from 1977-1979. A total of 629 occupied nests was examined. Highest nest densities (0.08 nest/km2) in the Missouri Coteau in 1979. Few nests were found on the more intensively cultivated Drift Plain. The breeding population was estimated in the study area each year using 148 sample plots [.hivin.x (mean) = 440 nesting pairs]. Pairs that nested on the ground selected only sites on rugged landforms in isolated areas, mostly end and dead-ice moraines. Land use within 1.0 km of nests was mostly (76.5%) pasture and haylands. About 95% of the area around ground nests was grassland. Tree and tower nesting birds appeared less sensitive to surrounding land use. Pairs on the intensive study block used a wide variety of nest sites including trees (63.6%), ground (20.9%), power line towers (8.0%), haystacks (5.4%) and rock piles, utility poles and shrubs (2.1%). Reoccupancy rates were higher (63.7%) for tower nests than for other sites. Nest success was related to the type of nest site and was highest (86.7%) in certain power line towers. Mean number of young fledged per occupied nest was highest (2.8) for ground nests. Summer storms were a major cause of loss; tree nests were particulary vulnerble. Richardson''s groundnests. Summer storms were a major cause of loss; tree nests were particularly vulnerable. Richardson''s ground squirrels (Spermophilus richardsonii) were taken as prey most often. Ferruginous hawks are closely associated with grasslands, particularly native prairie.