Published: August 1996
Author(s): Scott Richardson
North America's largest buteo, the ferruginous hawk, exists in low numbers in shrub-steppe and grassland regions of several eastern Washington counties. The state population, numbering between 50 and 60 nesting pairs occurs at the northwest edge of the species' breeding range. The hawk was listed as State Threatened by Department of Game Policy in 1983. In 1990, the Washington Wildlife Commission maintained the ferruginous hawk on the state list of Threatened species, a subcategory of protected status.
The size of the historic population cannot be determined. In 1997, a statewide population estimate of 20 pairs was published after nest searches in specific area. A few years later, an estimate of 40 pairs was presented after a survey effort concentrating on known territories. Recently, more comprehensive surveys suggest a population of 50 to 60 pairs. The proportion of occupied territories with at least one nestling was 72% in 1992, 82% in 1993, and 87% in 1994. At least 88, 87 and 82 chicks hatched in 1992, 1993 and 1994, respectively.
Ferruginous hawks generally arrive on their Washington breeding grounds during February or March. They nest on isolated trees, rock outcrops, and other platforms that provide unobstructed views. Nests are built of branches and bark shreds from nearby shrubs and often contain dried dung. Two to six eggs are laid, with a usual clutch size of three or four. Adults share duties during an incubation period that lasts about 32 days. Young fledge about 41 days after hatching, generally from late May to late July. Post-fledging dispersal is gradual, with young remaining near the nesting territory for a few weeks before migration. Washington's ferruginous hawks probably migrate to the southwestern and southcentral United States or Mexico for the winter.
The diet of Washington ferruginous hawks consist primarily of small to medium-sized mammals, such as pocket gophers, mice, and ground squirrels, but often includes birds, reptiles, and insects. Nesting territories may be situated for exploitation of a particular prey species upon which the hawks are largely dependent. The fate of nesting attempts can be affected by fluctuations in prey abundance. Some hawks may leave an area in response to low prey densities, leading to a lifestyle sometimes described as nomadic.
Persecution by early settlers reduced the numbers of ferruginous hawks in Washington and the United States. Recent pressures are frequently related to land-use practices. Conversion of shrub-steppe for agriculture or grazing has broadened the influence of human activity, reduced nesting opportunities, and lowered the diversity and abundance of prey species. Human populations in the traditional ferruginous hawk range encroach upon nesting areas and may limit breeding success or reoccupancy of territories.
To recover and maintain Washington's population of ferruginous hawks, sufficient shrub-steppe and native grassland must be preserved and disturbance to nesting areas must be reduced or eliminated. The ferruginous hawk will be considered for downlisting from State Threatened status when Washington supports a 5-year average of 60 breeding pairs distributed to reflect probable historic conditions.