Ferruginous hawk (Buteo regalis) COSEWIC assessment and status report: chapter 7

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Population sizes and trends

Search effort

Survey effort has varied considerably for Ferruginous Hawk surveys (Table 1), which affects the precision of population estimates. In 2006, about 15,400 km² of suitable habitat was searched for nests in Saskatchewan, including most of the known historical breeding sites; this represents about 12% of the species’ range in Saskatchewan (U. Banasch, pers. comm. 2006). Because the previous four surveys in Alberta had relatively low precision, three new procedures were implemented for the 2005 survey of Ferruginous Hawks in Alberta (Downey 2005). First, the survey quadrats were stratified into two strata (< 50% and ≥ 50% native prairie); second, the number of quadrats was increased, and third, the study area was decreased to what it was previously in the 1987-1992 surveys (Taylor 2003, Saunders 2005).

Abundance

In Canada, the densities of breeding Ferruginous Hawks are highest in Alberta (Table 1). However, breeding densities vary greatly depending on prey availability. For example, there was a five-fold difference in densities between surveys done in 1976-77 and 1984-88 and those done in 1987. Note that these surveys are not strictly comparable as they are based on different survey areas and searching intensities. Ferruginous Hawks occur at lower densities in Saskatchewan (Table 1) but over a larger area.

In Manitoba, the number of nesting pairs averaged 43.2 between 1987-2004 (range 11 in 1987 to 56 in 1991); the latest count was 42 in 2005 (K. De Smet, pers. comm.). In Saskatchewan, 900 breeding localities for Ferruginous Hawks have been documented; this includes records compiled during completion of the atlas (Smith 1996), and the addition of more recent records (A. R. Smith, pers. comm.). Assuming that there are probably many sites that are unknown, A. R. Smith (pers. comm.) estimates that there are at least 1000 sites in the province. In the Maple Creek study area there is a 50% occupancy rate (A. Smith pers. comm.), which suggests that there could be 500 pairs of Ferruginous Hawks in Saskatchewan. A second rough “guesstimate” for Saskatchewan is of 300-400 pairs (D. Zazelenchuk, pers. comm.).

The Saskatchewan survey undertaken in 2006 covered approximately 12% of the Ferruginous Hawk’s range in that province. Searches were made throughout the range, but were not done randomly; all known historical sites were visited, augmented by other areas of suitable habitat (U. Banasch, pers. comm.). This survey found 278 nests but because of the non random way the survey was undertaken it is difficult to extrapolate this figure into an estimate of the overall Saskatchewan population. Considering the fact that all historical sites were checked and 278 nests were found, it seems likely that the Saskatchewan population is less than 500 pairs.

The latest estimate for the Alberta population is 618±162 pairs, or 1236 adult individuals (Downey 2006).

These provincial estimates (Alberta–618; Saskatchewan–500; Manitoba–42) suggest that the entire Canadian population could be less than 1200 pairs, half that estimated in 1998 (Kirk and Hyslop 1998).

Fluctuations and trends

The Canadian range of Ferruginous Hawks has decreased by about half since the 1920s (Schmutz and Schmutz 1980.

Four sources provide data on trends in Canadian Ferruginous Hawk populations: 1) Long-term data on nesting productivity in Saskatchewan and Manitoba and population estimates in Alberta; 2) Breeding Bird Survey (BBS) results; 3) Migration counts from hawkwatches and 4) Christmas Bird Counts from the United States.

1. Surveys in Alberta in 1987 and 1992 suggested that the provincial Ferruginous Hawk population was 1,700 pairs (Schmutz 1987, 1993); however, by 2000 only about 731 pairs were found (Stepnisky et al. 2001, 2004). The latest estimate (2005) is of 618 + 162 pairs; this is considered a more precise estimate compared to the 2000 count, and is substantially reduced from the 1992 estimate (Downey 2005). As the survey was based on 93% of the Grassland Region, it is possible that some birds may have been missed in outlying areas but it is believed that this would add only 50 pairs (R. Quinlan in pers. comm. to Downey 2005). However, it is certain that the population in Alberta has been at a low since 2000, and has been declining since the early 1990s (Downey 2005). The population decline by about 64% between 1992 and 2005. Alberta population estimates are graphed in Figure 4.

   Ferruginous Hawks show considerable fluctuations in breeding success in response to changes in the abundance of ground squirrels. In Alberta, Downey et al. (2003) found a positive correlation between Ferruginous Hawk numbers and densities of ground squirrels. Although historical data on ground squirrels in Alberta is limited, based on sales of poison used by farmers for ground squirrels (Schmutz and Hungle 1989) it is believed that ground squirrel numbers were high during the late 1980s and early 1990s. Nesting densities around Hanna, AB rose slightly from 10.3±0.5SE per 100 km² in 1975-77 to 13.5±0.2 in 1986-90, then declined sharply to 4.4±0.6 in 2001-06 (Figure 4; Schmutz et al. 2008).

   In Saskatchewan, Houston and Zazalenchuk (2006) recently examined productivity of 1,433 Ferruginous Hawk nests between 1969-2004 in relation to ground squirrel numbers. Between 1967-1987 (Period 1) when ground squirrel numbers were high, productivity averaged 3.0 young per nest. Nests with four and five young averaged 32% and 4% respectively of all nests examined. When ground squirrel numbers plummeted between 1988-1996 (period 2), average productivity of Ferruginous Hawks fell to 2.7 young per nest. By contrast with period 1, nests containing four and five young made up only 18% and 1%, respectively, of all nests examined. During 1997-2002 (period 3), despite the fact that when ground squirrel numbers increased slowly and irregularly (reaching an estimated half of their initial numbers in 2003), productivity of Ferruginous Hawks averaged only 2.7 young per nest. Ferruginous Hawk productivity only started to increase in 2003-2005 when productivity again averaged 3.0 young per nest (Houston and Zazalenchuk 2006).

    

2. BBS results, although based on a relatively small sample size, suggest a stable or increasing population trend for the Ferruginous Hawk in Canada (Table 3, Figure 5), but these indices are based on low sample sizes.

3. According to the latest analyses from six hawkwatch sites in western North America, long-term declines have occurred in Ferruginous Hawk populations (Hoffman and Smith 2003). At two of these sites (Goshute Mountains, Nevada and Wellsville Mountains, Utah) passage rates increased until the early to mid-1990s, then decreased; long-term declines have occurred at the other two sites (Manzano Mountains, New Mexico and Lipan Point, Arizona). Passage rates were also below average for the other two migration sites since 1998 (Hoffman and Smith 2003).
4. Continental trends for the Ferruginous Hawk derived from the Christmas Bird Count data (1977-2001) indicate a significant quadratic trend (convex hill pattern, i.e. a bell-shaped distribution; P < 0.01; National Audubon Society 2005). A more recent analysis of the same data produced a significant increasing annual population trend of 2.1±0.7% throughout the species’ range over the last 40 years.

In summary, BBS data and Alberta surveys both indicate an increase in population during the 1980s, followed by a decline in the 1990s. The present population seems to be lower in Alberta now than it was in the early 1990s, but the national population trend is rather unclear because there has not been a series of comparable population surveys in Saskatchewan. Demographic studies in Saskatchewan, however, show a decline in nesting density, and numbers in Manitoba have declined as well.

Rescue effect

It is possible that, given the nomadic propensity of the Ferruginous Hawk, hawks could occupy vacated areas if native grassland habitat and an adequate prey base were available. However, little is known about the site fidelity of migratory populations or the extent of nomadism; there are few data on return rates of banded individuals (Bechard and Schmutz 1995).