Ferruginous hawk (Buteo regalis) COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

To the east of the Rocky Mountains, the Ferruginous Hawk is strongly dependent on native grasslands, whereas west of the Rockies, grasslands, shrub-steppes and other arid areas are heavily used (Bechard and Schmutz 1995). Aspen (Populus tremuloides) parkland, montane forests and areas of intensive agriculture are avoided. Based on the fact that the Richardson’s ground squirrel (Spermophilus richardsonii), which are the most important prey item of the Ferruginous Hawk, prefers areas with >30% cultivation and a grass sward height <30 cm (Downey et al. 2006), it is likely that such habitat is also preferred by Ferruginous Hawks; this has been confirmed by a positive correlation between hawk nesting densities and densities of ground squirrels (Downey et al. 2005). Although Ferruginous Hawk territories may contain cultivated areas, nesting sites are generally located in pasture or native grassland (Figure 3; Schmutz 1993, Bechard and Schmutz 1995). Over much of the species’ range, grasslands have been subject to destruction, degradation and fragmentation by urbanization, farming and industrial development (Bechard and Schmutz 1995). Ferruginous Hawks are very sensitive to habitat loss and are considered a native grassland specialist.

Figure 3. Frequency distribution of Ferruginous Hawk nests in relation to cultivation density on Alberta study plots (from Schmutz 1993).

Figure 3. Frequency distribution of Ferruginous Hawk nests in relation to cultivation density on Albertastudy plots (from Schmutz 1993).

Habitat trends

The distribution of the Ferruginous Hawk retracted at the edge of its range in Canada during the early 1900s because of agriculture and fire suppression allowing the invasion of trembling aspen into the remaining native prairie grassland (Houston and Bechard 1984, Schmutz et al. 1992). However, trembling aspen invasion is not an issue in the drier parts of the Ferruginous Hawk range. A negative relationship between the proportion of land under cultivation and site occupancy by Ferruginous Hawks has been demonstrated many times in several different geographical areas (e.g., Gilmer and Stewart 1983, Woffinden and Murphy 1989, Schmutz 1993, 1994). One reason postulated for this relationship is the fact that Ferruginous Hawks are very sensitive to disturbance during the nesting season. However, there is some evidence that the type of disturbance is important; disturbance from mining causes nest desertion and productivity is low near oil and gas wells (Olendorff 1993, Stepnisky et al. 2004). Railroads do not disturb nesting pairs, but primary and secondary roads do as evidenced by the fact that Ferruginous Hawks nest further away from them than do Swainson’s Hawks (Bechard and Schmutz 1995). The sensitivity of Ferruginous Hawks may cause pairs to avoid areas subject to disturbance or they may abandon nests leading to nest failure or they may relocate nesting territories elsewhere.

While Ferruginous Hawks are sensitive to disturbance, the fact that no significant change occurred in the proportion of cultivated land between 1982-2005 in the Alberta study population suggests that factors other than habitat loss to agriculture have caused declines in Ferruginous Hawk numbers (Downey 2005). These factors may include habitat loss to oil and gas, residential or other developments.

Habitat protection/ownership

Most suitable land for Ferruginous Hawks is on privately owned/operated ranchland where native range prevails (Schmutz 1999, Houston 2004). Almost 500 km² of suitable habitat is protected in Grasslands National Park.

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