Rusty blackbird (Euphagus carolinus) COSEWIC assessment and status report: chapter 7
Population Sizes and Trends
A variety of methods are used to survey Rusty Blackbird populations. Below, is a description of each method and its limitations for monitoring Rusty Blackbirds.
The Christmas Bird Count (CBC)
The Christmas Bird Count is a program that tracks North American bird populations in winter, and has been providing estimates of Rusty Blackbird wintering population trends for several decades (Sauer et al. 1996). Volunteers note all species found within a 24-km diameter circle on a single day between 14 December and 5 January (Sauer et al. 1996). The main advantage of this method is that it samples Rusty Blackbird populations throughout their wintering range in the U.S. (Sauer et al. 1996). One of the major disadvantages of this method is that Rusty Blackbirds may be underestimated because they mix with groups of similar species on the wintering grounds and, thus, may be difficult to detect.
The Breeding Bird Survey (BBS)
The Breeding Bird Survey is a program that surveys North American bird populations during the breeding season (Sauer et al. 2004). Bird abundance data are collected by volunteers at listening stations located along randomly selected roadside routes throughout North America (Downes et al. 2003). The BBS has several disadvantages in terms of monitoring Rusty Blackbird populations. One limitation is thatit samples well under one-third of the breeding range of this species, much of which is in less populated and inaccessible locations (Cyr and Larivée 1995). Also, BBSs are usually conducted in June, when Rusty Blackbirds are relatively quiet and, therefore, less detectable (C. Savignac pers. obs.).
The Canadian Migration Monitoring Program
The Canadian Migration Monitoring Program aims to track migratory passerines through a series of monitoring stations across Canada. The main activity carried out at these stations is bird banding and visual migration tracking. The main disadvantage of this program in terms of Rusty Blackbird monitoring is that relatively few of the stations get Rusty Blackbirds on migration and for those that do, it is often difficult to detect the birds because they are found in mixed flocks with similar species (C. Savignac pers. obs.).
Étude des populations d'oiseaux du Québec (EPOQ)
The Étude des populations d'oiseaux du Québec has been managing ornithological checklists produced by thousands of volunteers since 1969 and is the basic reference for determining Rusty Blackbird population trends in Québec (Cyr and Larivée 1995). The EPOQ database covers all regions south of latitude 52º north and includes all seasons (Cyr and Larivée 1995). The main disadvantage of this method is that it tends to cover mostly inhabited areas where access is easier. Nevertheless, this program is able to survey the Rusty Blackbird during its migration and can provide seasonal trends for the species (Cyr and Larivée 1995). Furthermore, the EPOQ database trends correlate roughly with those of the BBS, and can accurately highlight serious population declines (Dunn et al. 1996).
Ontario Breeding Bird Atlas (OBBA)
The work of the Ontario Breeding Bird Atlas between 1981-1985 (Cadman et al. 1987) and the current atlas (2001-2005) provides information on changes in the distribution of Rusty Blackbirds in the province in the 20 years between surveys.
Rusty Blackbird densities vary greatly according to region. Although many monitoring techniques do not adequately survey the Rusty Blackbird (Consortium Gauthier et Guillemet – G.R.E.B.E. 1991; Drapeau et al. 2000; Schieck and Hobson 2000; M.-A. Villard pers. comm.; S. Van Wilgenburg pers. comm.), the densities reported below provide some information on the range of densities that have been reported during the breeding season.
In eastern Canada, breeding bird surveys show that Rusty Blackbird densities vary from region to region, but are generally low. In Newfoundland and Labrador, densities of 6 birds/km² have been reported, while in Nova Scotia 2/km² have been observed (Erskine 1977). In Québec, recent surveys have found densities of 0.2/km² in riparian zones of lakes and hydroelectric reservoirs from James Bay to Labrador (220 transects of 400 m over 500 km²; J. Gauthier unpubl. data), 0.6/km² in riparian strips within Québec’s boreal forest (Darveau et al. 1995), 17.4/km² in the riparian areas of streams of the St. Lawrence Valley (Larue et al. 1995) and 100/km² in bordering coniferous forests and peatlands in the Grande-Baleine River watershed of northwestern Québec (Consortium Gauthier et Guillemet – G.R.E.B.E. 1991).
In the northwest of the Rusty Blackbird’s breeding range, densities appear to be generally higher than those reported in eastern Canada. For example, surveys conducted in the early 1970s in riparian areas along the Mackenzie River, NWT, found densities ranging from 15 to 100/km² (Schweinsburg 1974; Gunn et al. 1975, 1977). More recent studies conducted in several types of wetlands in northern Saskatchewan found densities ranging from 2 to 31/km² (Hobson et al. 2000). In contrast, a 2003/2004 survey by Ducks Unlimited covering over 150,000 km² of northern Saskatchewan and Alberta found only seven Rusty Blackbirds (J. Morissette pers. comm.). Breeding bird surveys conducted in the Hudson Bay lowlands of northern Manitoba found densities of 20/km² (Gillespie 1982).
The total Rusty Blackbird population is difficult to estimate because of the small number of studies conducted in riparian areas and survey techniques that are not suitable for surveying Rusty Blackbird populations. However, some population estimates have been made. Using data from the Atlas of Breeding Birds of the Maritime Provinces, Erskine (1992) estimated the Rusty Blackbird populations of New Brunswick, Nova Scotia and Prince Edward Island at 13,400 + 3,200, 22,000 + 4,000, and 1,600 + 1,000 individuals, respectively. An estimate of the Canadian population, which comes from the Canadian BBS and the Canadian Breeding Bird Census Database (Kennedy et al. 1999), suggests a population of approximately 1.4 million individuals. This estimate is based on Canada including approximately 70% of the global breeding population of 2 million individuals (Blancher 2003). Another estimate of the Canadian population comes from an extrapolation of counts made at roosts across the U.S. during the winter of 1974-75 (Meanley 1976). This nationwide survey suggested that the global population at that time was approximately 1,103,000 birds (number of birds counted + number missed given 35 main roosts were not surveyed). Assuming that 70% of the global population breeds in Canada (Blancher 2003), then the Canadian population in the mid-70s was approximately 772,100 birds. Assuming that 85.7% of the population has been lost over the last 38 years based on Christmas Bird Count trends (see below), the current Canadian population is estimated at 110,400 individuals. Thus, the Rusty Blackbird population in Canada ranges roughly from a minimum of 110,400 to a maximum of 1.4 million individuals.
Fluctuations and trends
The Christmas Bird Count (CBC)
Despite its limitations (see above), the CBC is the best method for determining Rusty Blackbird population trends because it surveys most of the wintering range in the U.S. CBC data show a significant decline of -5.1%/year between 1966 and 2003 (Figure 2; Table 2). Given this rate of decline, the Rusty Blackbird population will have decreased by 85.7% over the last 38 years (Niven et al. 2004). The decline in the most recent 10-year period (1994-2003) has been approximately -2.1%/year (Figure 3, Table 2). At this rate, the population will have decreased by 18.3% over the last decade.
Figure 2. Population trends for the Rusty Blackbird taken from the North American CBC (circles) and BBS (diamonds) surveys for 1966-2003 (taken from Niven et al. 2004). Credible intervals (the Bayesian equivalent of confidence intervals) for the CBC are shown as bars.
ns = non-significant.
Figure 3. Trends for the Rusty Blackbird taken from the North American CBC for 1994-2003. Credible intervals (the Bayesian equivalent of confidence intervals) are shown as bars. (data courtesy of Dan Niven).
The Breeding Bird Survey
Based on BBS data, the Rusty Blackbird population in North America has shown a significant decline of -9.97%/year over the last 38 years (1966-2003; Figure 2, Table 2). At this annual rate of decline, the global Rusty Blackbird population will have decreased by 98% since 1966. In Canada, Rusty Blackbird populations have shown a significant decline of -10.3%/year over a similar time period (1968-2002; Table 2) and a non-significant decline of -6%/year for the most recent 10-year period (1993-2002; Table 2). For the majority of Canadian provinces, a precise picture of trends cannot be obtained because of low sample sizes (Downes et al. 2003). Of seven provinces with BBS information, however, all showed negative trends in the 1968-2003 period.
The Canadian Migration Monitoring Program
Of the 22 monitoring stations, only three record sufficient numbers of Rusty Blackbirds to produce trends. The Mackenzie Nature Observatory station in northeastern British Columbia, showed a significant decline of -24.3%/year in migrating populations of Rusty Blackbirds between 1996 and 2002, while Thunder Cape Bird Observatory showed a non-significant decline of -3.76%/year between 1995 and 2002 (Table 2). Neither period is long enough to use the most appropriate analysis of migration data, which includes adjustment for weather effects on numbers recorded (E. Dunn pers. comm.). Finally, analyses based on 13 standardized counts (9131 hours of observation) from the Observatoire d’oiseaux de Tadoussac between 1996 and 2005 suggested a negative correlation (R² = 0.45) between year and the abundance of Rusty Blackbirds observed on the fall migration. Data from this site also suggest that population size might fluctuate over a 5-year cycle, which could also increase the vulnerability of the species (B. Drolet pers. comm.).
Étude des populations d'oiseaux du Québec
The EPOQ database showed a decline of -4.6%/year between 1970 and 1995, which is equivalent to a total decline of 59% since 1970 (Cyr and Larivée 1995). A more recent analysis of the database for the period 1970-2003 showed further decline in Rusty Blackbird numbers, but at the lower rate of -2.7%/year (Table 2).
The OntarioBreeding Bird Atlas
A comparison of data from the second Ontario Breeding Bird Atlas (2001-2005) with those from the first atlas (1981-1985, Cadman et al. 1987) show a significant decline in the number of "matched" squares (i.e., well-covered squares where the species was reported in both atlases) of 12% in the northern shield area and 4% in the southern shield (M. Cadman pers. comm.). Interestingly, there was a non-significant increase in the number of matched squares in the Hudson Bay Lowland, suggesting that this may be an important breeding area for this species (M. Cadman pers. comm.).
Although there is no direct evidence of immigration from the U.S., some immigration almost certainly occurs, particularly from Rusty Blackbird populations in eastern Alaska, where there are relatively large breeding populations (International Rusty Blackbird Technical Group 2005). Immigration from these areas would probably affect only populations in northern British Columbia, Northwest Territories and the Yukon, however. Rusty Blackbird populations from the New England states are probably too low to provide a rescue effect for populations in the Maritimes, southern Québec or Ontario.
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