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The California Gull typically nests in colonies on islands in lakes, making it difficult to survey with the roadside Breeding Bird Survey (Winkler 1996). Similarly, because the species forms large flocks in winter, Christmas Bird Counts (CBC) can suffer from counting error. In both cases, the precision of the trend estimate from both surveys is poor. Despite these limitations, both surveys suggest that the species has increased in abundance since about 1970 in Canada and, for CBC, throughout its range, though the magnitude of the increase is uncertain.
Additional information on: Breeding Bird Survey (BBS) - Canadian analysis
Additional information on: Christmas Bird Count (CBC)