Population Trend (PT-c) indicates vulnerability due to the direction and magnitude of recent changes in population size. Like the threats scores, PT-c actually reflects trends only within North America, even for species with ranges that extend beyond the continent. It is therefore a continental, rather than a global, score. Species that have declined by 50% or more over 30 years are considered most vulnerable, whereas species with increasing trends are least vulnerable.
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Breeding Distribution was calculated by determining the area (km2) or for coastal breeders, length (km) of linear coastline occupied by breeding-aged individuals during the breeding season, using range maps for the species in well-known field guides (e.g., Harrison 1983 National Geographic Society 1987, Howell and Webb 1995), as well as other sources (e.g., NatureServe).
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[Population size] Scores were assigned using population estimates derived primarily from abundance data collected by the North American Breeding Bird Survey (BBS), extrapolated after various adjustments to range size outside of BBS coverage; but other data on abundance were used when appropriate (Rich et al. 2004, Appendix B; Rosenberg and Blancher 2005).
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The primary source of trends was the BBS, but Christmas Bird Count (CBC) or specialized data sources were used where these were the best available breeding or non-breeding data on North American population trends. In some cases, particularly for extirpated or possibly extinct species, historical trends were considered. Where empirical data did not exist, PT-c was assigned by expert opinion, using the qualitative definitions below as guidelines.
