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Red-shouldered Hawk (Buteo Lineatus)

Population Sizes and Trends

Search Effort

Data on Red-shouldered Hawk abundance and population trends are collected through several different programs in Canada. A brief description of methodology, potential biases and utility of the program in monitoring changes in Red-shouldered Hawk populations are discussed below.  

Breeding Bird Survey

The Breeding Bird Survey is a roadside survey, conducted annually in mid-June in road-accessible locations throughout North America. Although the BBS takes place throughout the Red-shouldered Hawk’s range, it has limited value for monitoring Red-shouldered Hawks because it takes place well after the main period for this species’ conspicuous calling and display flights. For this reason, the species is generally reported in low numbers. The relative abundance of Red-shouldered Hawks on Canadian routes from 1966–2003 was only 0.03 birds/route. Trends for Red-shouldered Hawks in both Canada and for all of North America calculated from BBS data should be viewed with some caution because they have a credibility measure category that reflects data with a deficiency (Sauer et al. 2004). 

Ontario Red-shouldered Hawk and Spring Woodpecker Survey

In Ontario, Red-shouldered Hawks are surveyed through this specialized survey that uses tape playback to elicit vocal responses from Red-shouldered Hawks during the period when they are most vocal (17 April–7 May; Badzinski 2004). Mean detection rates for this survey are typically around 3.0 birds/route (Badzinski 2004). Power analyses conducted using the Red-shouldered Hawk survey data show that 31 routes need to be surveyed annually in order to detect a 20% change over 10 years (Francis 1999). From 2000–2004, the number of routes completed varied from a low of 52 routes in 2004 to a high of 64 routes in 2001, which is more than adequate to detect a 20% change over 10 years. Like all roadside surveys, this survey has some biases. Detection rate likely varies among observers, differences in the quality of the stereo used for playback may affect response rate, and forest interior habitats may be underrepresented. Despite these limitations, the use of taped calls and the earlier timing of this survey result in a higher proportion of birds being counted at each survey station. This method is, therefore, more appropriate for monitoring population trends and forms the basis for an Ontario population estimate.

Breeding Bird Atlases

Breeding bird atlases collect data on the distribution and abundance of Red-shouldered Hawks, but are only done once every 20 years. The first breeding bird atlas in Quebec ran from 1984-1989, the Maritime Atlas from 1986–1990 and the Ontario Atlas from 1981–1985. The second Ontario Breeding Bird Atlas began in 2001 and will be completed in 2005. Data from breeding bird atlases are useful in tracking changes in distribution of species over time. If point counts are included as part of atlas methodology, data can also be used to examine relative abundance and population size. It is important, however, that changes in observer effort across atlas periods be considered. 

Hawk Migration Monitoring

There are several hawk watch stations in Ontario, Quebec and the neighbouring states that monitor trends in numbers of migrant Red-shouldered Hawks in spring and fall. Observers at the stations count migrating raptors using a standardized protocol that includes a record of the number of observer hours. Appropriately analyzed, these data can be used to determine trends in numbers of birds of a given species over time (Hussell and Brown 1992). Migration data can be used to estimate minimum population size; however, without knowing the percentage of the population that is being counted, and the ultimate destination of the birds, accurate population estimates are not possible.

Bird Checklist Program

The Association Québécoise des Groupes d'Ornithologues (AQGO) operates a checklist program in Quebec - Étude des Populations d’Oiseaux du Québec (ÉPOQ), which is North America’s longest-running and largest checklist program. The Quebec checklist program is a compilation of birders’ observations from birding trips at any time of year and at any location. Checklist data can be used to investigate bird population trends (Cyr and Larivée 1995); however, trends generated from the checklist data tend to be positively biased (Dunn et al. 1996). This positive bias occurs because of improving birding skills over time or shifts by birders to more productive areas as species decline in previously-favoured spots (Dunn et al. 1996). Despite these potential biases, negative trends are considered reliable indicators of true declines. From 1969-2003, 118 484 checklists with 4 053 Red-shouldered Hawk observations were submitted for southern Quebec, and 196 516 checklists with 3 259 observations for central Quebec (F. Shaffer pers. comm.). 

Christmas Bird Count

Most Red-shouldered Hawks winter in the United States, so appropriately analyzed Christmas Bird Count data from the United States are useful in examining overall continent-wide trends and winter distribution. The Christmas Bird Count is conducted in more than 1800 locations across Canada, the United States and Latin America. Here, observers attempt to count all birds within a 24-km diameter circle on a selected day between 14 December and 5 January.

 

Abundance

Over the last 20 years, there have been a variety of population estimates for Red-shouldered Hawks in Canada. Risley (1982) originally estimated a total Canadian population of Red-shouldered Hawks at 468 pairs. Austen et al. (1994), using data from the first Ontario Breeding Bird Atlas, then estimated the Ontario population of Red-shouldered Hawks at 824 – 2372 breeding pairs. Kirk et al. (1995), using data from a variety of sources, estimated the Red-shouldered Hawk population in Canada at 2000 – 5000 pairs, which was considerably higher than previous estimates.

The most current population estimate (2004) for Red-shouldered Hawks in Ontario, derived from Red-shouldered Hawk and Spring Woodpecker Survey data (1991–2004) is 5850 breeding pairs (P. Blancher pers. comm.). Data were screened to exclude routes of questionable observer ability, and the few data from routes north of 47 or south of 43 degrees latitude were also excluded. Counts of Red-shouldered Hawks were determined for each survey station across all years surveyed, and then averaged across stations within routes, across routes within 10 x 10 km breeding bird atlas squares, and then across atlas squares within each atlas block (up to 100 km on a side) in order to avoid overweighting any part of the breeding range. Counts at stations were assumed to include all hawks within 500 m of the observer. Average counts per block were then extrapolated to the forested parts of each survey block to obtain a population estimate. The estimate is based on several assumptions: it assumes that all habitat at each survey station is suitable habitat for hawks, and not of lower quality than off-road forest habitat, it assumes that all birds within a 1 km diameter circle are detected, whether male or female (or missed birds are offset by birds drawn in from farther away), and it assumes that the number of hawks south of 43 and north of 47 degrees latitude is negligible.

There are no recent population estimates for the species in Quebec, but, data from the Breeding Bird Atlas (1984–1989) suggested a minimum of 400 pairs, and a maximum of 1000 pairs (F. Shaffer pers. comm.). Similarly, the Maritime Breeding Bird Atlas (1986–1990) suggested the New Brunswick Red-shouldered Hawk population was less than 20 pairs (Erskine 1992). However, observations of Red-shouldered Hawks over the past 15 years and results of recent surveys suggest that this underestimates the breeding population in New Brunswick (D. Sabine pers. comm.). Based on these provincial estimates, a conservative population estimate for Canada would be 6270 pairs (5850+400+20).

 

Fluctuations and Trends

BBS data show no significant changes in Red-shouldered Hawk populations in Canada from 1980–2003 or from 1994–2003 (0.65%/year, N=19, P=0.85; -2.8%/year, N=11, P=0.61, respectively) and a significant increase in the United States from 1994–2003 (2.7%/year, N=631, P<0.005; Sauer et al. 2004). In Ontario, the Red-shouldered Hawk and Spring Woodpecker Survey results suggest that the population has not changed significantly from 1991–2004 (-0.3%/year, 95% confidence interval: -1.5 to 0.9%, P=0.60; Figure 4; Badzinski 2004).


Figure 4: Estimated Annual Indices for Red-shouldered Hawks Using Data from the Ontario Red-shouldered Hawk and Spring Woodpecker Survey (1991–2004)

Figure 4. Estimated annual indices for Red-shouldered Hawks using data from the Ontario Red-shouldered Hawk and Spring Woodpecker Survey (1991-2004). 

Indices were derived from a generalized linear model assuming Poisson residuals and a log-link function. 95% confidence limits refer to differences from 2004, which was chosen as the baseline year. Year-to-year comparisons based on post hoc contrasts were significantly different as marked: + P<0.10, * P<0.05, ** P<0.01.

Data from the second Ontario Breeding Bird Atlas suggest a stable distribution in Ontario, although there have been some local changes (Figure 2). During the first Ontario Breeding Bird Atlas (1981–1985), Red-shouldered Hawks were found in 384 squares (10 km x 10 km) throughout Ontario. In the first four years of the second Breeding Bird Atlas (2001–2004), Red-shouldered Hawks have been found in 427 squares (2nd Ontario Breeding Bird Atlas unpublished data; Table 1). This apparent increase should be viewed with some caution, however, because of differences in search effort between the two atlases. The Quebec and Maritime Breeding Bird Atlases were completed in 1989 and 1990 respectively, so there are no data available on recent population changes from these sources. 

 

Table 1: Red-shouldered Hawk Breeding Evidence Collected During the First Ontario Breeding Bird Atlas (1980-1984) and the First Four Years of the Second Ontario Breeding Bird Atlas (2001–2004)
Atlas Squares (10 km x 10 km)
reporting Red-shouldered Hawks
1st Atlas2nd Atlas
Squares with confirmed breeding 6687
Squares with probable or possible breeding318340
Total squares384427

Data from hawk watch stations in Ontario suggest stable or increasing numbers of birds in both spring and fall migration counts. Although these data have been corrected for variation in observer effort, they have not been log transformed and so should be interpreted with caution. Spring migration data from the Niagara Peninsula Hawkwatch in Beamer, Ontario show an apparent increase from 1980–1990, followed by an apparent decline from 1990–2004 (Figure 5). Fall migration data from Holiday Beach Migration Observatory in southwestern Ontario (1980–2004; Chartier and Stimac 2002) and Cranberry Marsh Raptor Watch in Grimsby, Ontario (1990–2003) show no change in numbers of migrating Red-shouldered Hawks. Data from hawk migration stations in Quebec (which were appropriately analyzed), however, show a significant linear increase in both spring and fall from 1980–2004 (P<0.001; Shaffer and Dionne 2004; Figure 6). Data from these numerous hawk watch stations suggest that the population of Red-shouldered Hawks has been stable over the last 10 and 20 years with an apparent local increase in Quebec.


Figure 5: Numbers of Migrating Red-shouldered Hawks/Hour Observed at the Niagara Peninsula Hawkwatch During Spring 1980–2003

Figure 5.  Numbers of migrating Red-shouldered Hawks/hour observed at the Niagara Peninsula Hawkwatch during spring 1980-2003.


Figure 6: Numbers of Migrating Red-shouldered Hawks/Hour Observed at Ste. Anne-de-Bellevue (spring) and St. Stanislas-de-Kostka (fall) 1980–2004

Figure 6. Numbers of migrating Red-shouldered Hawks/hour observed at Ste. Anne-de-Bellevue (spring) and St. Stanislas-de-Kostka (fall) 1980-2004.

The dotted and full lines represent the trend calculated for the spring and fall season, respectively.

The ÉPOQ Checklist Program suggests that the numbers of Red-shouldered Hawks in Quebec have increased significantly from 1990–2003 (Figure 7). Although ÉPOQ trends are positively biased (Dunn et al. 1996), it is highly unlikely that there has been any important decline in Quebec.

Summary of Fluctuations and Trends

Data from a variety of sources suggest that the Red-shouldered Hawk population in Canada has been stable over the past 10 and 20 years, with some local increases. There is no evidence of population declines. Data from Ontario show a stable population over these time periods whereas data from Quebec suggest that the Red-shouldered Hawk population may have increased.


Figure 7: Trends in Numbers of Red-Shouldered Hawks Reported to the Étude des Populations d’Oiseaux du Québec (ÉPOQ) Checklist Program

Figure 7. Trends in numbers of Red-shouldered Hawks reported to the Étude des Populations d’Oiseaux du Québec (ÉPOQ) Checklist Program.

Data show a significant increase in Red-shouldered Hawks in both the northern and southern regions of the species’ range in Quebec.

 

Rescue Effect

Over the last 20 years, both BBS data (Sauer et al. 2004) and Christmas Bird Count data (National Audubon Society 2002) suggest that the Red-shouldered Hawk population in the United States is stable or increasing throughout much of its range (Sauer et al. 2004). Thus, the U.S. population of Red-shouldered Hawks appears healthy and capable of providing immigrants to Canada. Because this species is migratory, it is physiologically capable of dispersing to new areas, but there is little information available on dispersal distances and rates of natal philopatry. There is no information on whether birds hatched in the United States will emigrate to Canada.