COSEWIC Assessment and Status Report on the Common Nighthawk in Canada
- Assessment Summary
- Executive Summary
- COSEWIC History, Mandate, Membership and Definitions
- Lists of Figures and Tables
- Species Information
- Population Sizes and Trends
- Limiting Factors and Threats
- Special Significance of the Species
- Existing Protection or Other Status Designations
- Technical Summary
- Acknowledgements and Authorities Consulted
- Information Sources and Biographical Summary of Report Writer
Population Sizes and Trends
Breeding Bird Survey (BBS)
The BBS is a large-scale monitoring program that surveys North American bird populations during the breeding season (Sauer et al. 2005). Bird abundance data are collected by volunteers, who record all birds seen and heard within a 400 m radius circle at stops positioned every 800 m along roads (Downes et al. 2005). Although the BBS can track Common Nighthawks because the species is highly visible (Poulin et al. 1996; Sauer et al. 2005), it is considered to have only Fair accuracy for establishing population indices (Rich et al. 2004). One limitation of this method for tracking Common Nighthawks in Canada is that regions, such as the boreal forest, will not be as well represented as the more southerly and populated parts of the breeding range. Additionally, because the birds are crepuscular they tend to be detected only on the first stops of a BBS route on a given day.This limitation is somewhat countered by the fact that the species is detected on a large number of routes, providing a good sample size for trends (P. Blancher pers. comm. 2006).
Étude des populations des oiseaux du Québec (EPOQ)
In Quebec, the EPOQ database, which manages ornithological checklist data provided by thousands of volunteers since 1969, is the tool of choice for examining bird population trends in Quebec (Cyr and Larivée, 1995). The EPOQ database primarily covers regions (i.e. St. Lawrence lowlands) south of the 52nd parallel 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.
Ontario Breeding Bird Atlas
The Ontario Breeding Bird Atlas surveyed breeding bird populations in Ontario during the periods of 1981-1985 and 2001-2005. It provides information on changes in the distribution of Common Nighthawks in the province in the 20 years between surveys (Cadman et al. 1987; Ontario Breeding Bird Atlas, 2006). Population trends are determined by comparing the percentage of 10 X 10 km squares reporting Common Nighthawks between the two time periods and also by comparing the number of squares per 100 km x 100 km block divided by the total number of squares/block surveyed.
In British Columbia, Alberta, Saskatchewan and Manitoba, information on Common Nighthawk trends comes mostly from local ornithological knowledge (i.e. notes and observations from amateur ornithologists, see Manitoba Avian Research Committee, 2003) and includes counts of migrating birds as they pass fixed stations in Manitoba (Taylor, 1996) and mapping of breeding bird territories in Saskatchewan (Wedgwood, 1973).
According to BBS abundance estimates (after Rich et al. 2004), the current population of Common Nighthawks in Canada, is approximately 400,000 breeding adults or 200 000 breeding pairs (P. Blancher, pers. comm. 2007).
Other than the estimates provided by the BBS, very few studies exist which assess the abundance of the Common Nighthawk in Canada. Data on the density of the species are only available for the Saskatoon and Cluff Lake areas in Saskatchewan. Reported densities range from a minimum of 0.03 male/ha in urban areas around Saskatoon (Wedgwood, 1973) to 0.11 male/ha (33 males/24 stops) in boreal forest dominated by a 25 year old burn in the Cluff Lake area (C. Savignac, unpubl. data 2005).
Breeding Bird Survey
In the United States, where most of the breeding population of Common Nighthawks occurs, the long-term BBS data show a significant decline of 1.83% per year (n = 1498 routes, P < 0.00) between 1968 and 2005 and, on the short-term, a significant decline of 1.58% per year (n = 995, P = 0.02) between 1995 and 2005 (Sauer et al. 2005).
In Canada, long-term BBS data show a significant decline of 4.2% per year (n = 312 routes, P < 0.05) between 1968 and 2005, which corresponds to an 80% decline in population over this time period (Downes et al. 2005; Figure 2). In the most recent 10-year period (1995-2005), BBS data show a significant decline of 6.6% per year (n = 164 routes, P < 0.05), which amounts to a 49.5% decrease in the population (Downes et al. 2005). BBS routes in parts of the boreal forest also show declines on both the long and short-term (Boreal Taiga Plains: 1968-2005: -13.7%/year, n = 47 routes, P <0.05; 1995-2006: -17.5%/year, n = 23 routes, P > 0.05; Boreal Hardwood: 1968-2005: -6.8%/year, n = 40 routes, P <0.05).
Étude des populations des oiseaux du Québec
The EPOQ database for Quebec shows a significant long-term decline of 0.24% per year (Y = -0.0024x + 4.9, P £0.01) between 1970 and 2004 (Larivée, 2005; Figure 3) and a non-significant short-term decline of 0.20% per year (Y = -0.002x + 4.6, P = 0.25) between 1991 and 2004 (Larivée, 2005).
Ontario Breeding Bird Atlas
A comparison of data from the first (1981-1985) to the second (2001-2005) atlas period shows a decrease in the number of squares reporting a Common Nighthawk between the two survey periods (1981-1985: 38% of squares, 2001-2005: 21%, Cadman et al. 1987; A. Darwin unpubl. data 2005). Similarly, the number of 100 x 100 km blocks where there were fewer squares with Common Nighthawks in the second atlas than the first (n = 115) was greater than the number of blocks where the squares with nighthawks had increased (n = 14) or stayed the same (n = 1) (Wilcoxon Sign test 2-tailed = -8.8, P £0.001; A. Darwin, unpubl. data 2005).
From Downes et al. 2005.
Recent results show significant declines between the two atlas periods in the Southern Shield Region, one of the population strongholds for this species in Ontario, of 31% and declines in the Carolinian (14%), Simcoe-Rideau (21%), and Northern Shield (20%) Regions (L. Friesen, pers. comm. 2007). The only region that did not report a significant decline was the Hudson Bay Lowlands, lying at the extreme northern edge of the species’ range (L. Friesen, pers. comm. 2007).
Although the species is still relatively abundant and widely distributed throughout the province, data coming primarily from the ornithological community of Manitoba indicate that the species has declined in several urban centres, including Winnipeg (Manitoba Avian Research Committee, 2003; P. Taylor pers. comm. 2005). Visual counts of migrating birds (probably from the boreal forest) in the Pinawa area suggest that the species declined by 75% between the periods 1976-1981 and 1992-1997 (Taylor, 1996; P. Taylor pers. comm. 2005; Figure 4). However, counts increased during the 2000-2005 period (P. Taylor pers. comm. 2005).
The comparison of an exhaustive Common Nighthawk survey in Saskatoon from 1971 to 1990 suggested a decline in breeding populations of 58% (i.e. 68 territories in 1971 vs. 28 in 1990, Wedgwood, 1991).
Counts during the periods 1992-1997 (n=51) and 2000-2005 (n=24) were more frequent and systematic than in 1976-1981 (n=19). The values on the x axis are individual counts assigned a percentile ranking between 0 and 100 to normalize the x-axis and allow comparisons across the three time periods (P. Taylor, unpubl. data).
Historical references to Common Nighthawk abundance are also consistent with the decline suggested by the above survey data. For instance, in the mid-1800s in the Montreal area, the species was considered a common resident that nested on the gravel rooftops of city houses (Wintle, 1882, 1896 in Ouellet, 1974). In the 1970s, the species was also considered a common summer resident in the Montreal and Montérégie areas and nested in small numbers in all cities and towns, although the species was not as common as in the 1800s (Ouellet, 1974). More recently, a significant decline in the species has been reported in these areas and in several other towns of the St. Lawrence Valley, notably Rimouski, Quebec City and Gatineau (J. Larivée pers. comm. 2005; J. Gauthier pers. comm. 2005).
In the early 1900s in Ontario, the species was considered abundant by Macoun and Macoun (1909). A decline was first reported in the early 1970s (Goodwin and Rosche, 1970, 1974).
In summary, both long and short-term BBS data, including routes in the boreal forest, show significant declines in the abundance of Common Nighthawk populations. Further, counts from Manitoba, which likely sample birds on migration from the boreal forest, have also shown long-term declines. These results are consistent with provincial surveys (e.g. Ontario Breeding Bird Atlas) that also show declines in nighthawk abundance. Although it is not clear what portion of the population occurs outside the populated areas (e.g. the boreal forest), the evidence suggests) that the observed declines are widespread across the breeding range.
BBS data for the United States, the potential source of immigrants for Canada, also show significant long and short-term declines in nighthawks (see above).
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