McCown's longspur (Calcarius mccownii) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Search effort and uncertainty

Data on McCown's Longspur population sizes and trends are collected by three main survey methods. Below is a description of each method and its limitations in monitoring McCown's Longspur populations.

Breeding Bird Surveys (BBS)

The Breeding Bird Survey is an annual survey conducted in mid-June throughout Canada and the United States. Volunteers travel along randomly selected roadside routes and record all birds heard or seen at listening stations located along the route (Sauer et al. 2003). One limitation of the BBS for monitoring McCown's Longspurs is that some of the most important routes for this species are surveyed irregularly. For instance, the three routes reporting the highest densities of McCown's Longspurs in Saskatchewan (Swift Current, Tuberose, and Tyner) have not been surveyed since 1995, 1973, and 1971, respectively (Sauer et al. 2003). Additionally, because of their limited distribution McCown's Longspurs are detected on few BBS routes (Sauer et al. 2003, Dale et al. 2005).

Grassland Bird Monitoring (GBM)

The Grassland Bird Monitoring program was developed to improve coverage of endemic grassland species and/or species "at risk". GBM surveys use an intensified BBS methodology with increased monitoring coverage in the 19 degree blocks where grassland birds and remaining grassland habitats are concentrated. The BBS route selection criterion of nearest secondary or better road is modified to accept the nearest passable road. Otherwise, route selection and bird data collection adhere to the BBS methodology (Dale et al. 2005).

Preliminary analyses suggest that GBM surveys are more effective than BBS in detecting grassland species (Dale et al. 2005). However, because the GBM program was begun in 1996, there are few data years available for analysis.

Christmas Bird Counts (CBC)

The Christmas Bird Count is an annual survey conducted in Canada, the U.S. and Latin America (Sauer et al. 1996). Volunteers count all the species found within a 24 km diameter circle on a pre-selected, single day between 14 December and 5 January (Sauer et al. 1996). One of the limitations of this method is that variation in weather conditions and the number and skill of observers across years introduce significant variability. CBC data from the southern U.S. and northern Mexico provide an estimate of the global population of this species as they winter in these areas.

Additional sampling concerns

McCown's Longspurs exhibit erratic population fluctuations and unpredictable occupation throughout much of their Canadian breeding range (With 1994a), possibly because they are at the edge of this range (Curnett et al. 1996). These erratic fluctuations make it difficult to effectively determine McCown's Longspur population trends, especially over relatively short time periods. High variation in the data, coupled with small sample sizes, can result in low statistical power (Dale et al. 2005) and increase the chance of a false negative (Type II Error).

Abundance

Extrapolating from BBS data, the North American population of McCown's Longspurs is estimated at approximately 1,100,000 breeding birds and the Canadian population at approximately 375,000 birds or 34% of the North American population (P. Blancher, pers. comm.). The accuracy of these population estimates is considered poor, however, and the true population of McCown's Longspurs in Canada is likely between 100,000 and 1,000,000 individuals (P. Blancher, pers. comm.).

Fluctuations and trends

Breeding Bird Surveys

Based on long-term BBS data from Bird Conservation Region 11, which includes all of the species’ range in Canada, there was an annual rate of decline of -12.9%/ year (n=33, 0.05<p<0.15) between 1968 and 2002 (CWS 2005, Figure 3). At this rate, the population will have decreased by approximately 98% since the late 1960s. This trend does appear, however, to have slowed over the last decade, with an annual rate of decline of -0.2%/year (p>0.10) between 1996 and 2004. Based on this rate of decline, the population will have decreased by approximately 1.5% during this recent period.

Grassland Bird Monitoring

Based on GBM surveys, which are conducted in the core of the range where habitat is relatively intact, McCown's Longspur populations in Canada showed an annual rate of decline of -0.40%/year (95% CI: -20.0, 23.9) between 1996 and 2004 (B. Dale, pers. comm.). Based on this rate of decline, the population will have decreased by approximately 3% during this period.

Christmas Bird Count

CBC data, based on surveys conducted on the wintering grounds in the U.S., showed a decline of -2.5.%/year (Credible Intervals: -12, 8.9) between 1966 and 2004 in the global population of McCown's Longspurs (D. Niven, pers. comm.).

Summary

Overall, analyses based on long-term (i.e., 1968-2002) survey data show a strong decline in McCown's Longspur numbers in Canada. This trend appears, however, to have slowed and analyses based on surveys conducted between 1996 and 2004 suggest that the population has been stable over the last decade.

Rescue effect

BBS data from the 1990s in the U.S. suggest a population of approximately 700,000 birds (Rich et al. 2004). This population showed a non-significant increase of 1.2%/year between 1966 and 2003 (n=41, p=0.60, 95% CI: -3.4, +5.8; Sauer et al. 2003). Therefore, McCown's Longspurs breeding in the United States could possibly act as a source population of immigrants to Canada.

Language selection

Search