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Retrospective Comparison of the Occurrence and Abundance of Rusty Blackbird in the Mackenzie Valley, Northwest Territories

机译:西北地区麦肯齐山谷中生锈黑鸟的发生和丰度的回顾性比较

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Rusty Blackbird is listed as a species of “special concern” by the Committee On the Status of Endangered Wildlife In Canada, and has shown steep population declines in recent decades. Forty-five locations with historical survey data from the 1970s in the Mackenzie Valley, Northwest Territories, Canada were revisited in 2006 to check for changes in the occurrence or abundance of Rusty Blackbird. Our retrospective analysis revealed a number of analytical challenges for such comparisons that we describe. The number of lakes on which this species occurred does not appear to have declined significantly in the past three decades when a correction for survey duration was applied. The range-wide decline of 5.1%/yr based on Christmas Bird Count data would have resulted in 2006 occupancy at ≈5 lakes. We estimate that with correction this would have increased to ≈26. However, naive or unadjusted analyses with a Chi-squared test showed a significant decline. A simulated resampling of the historical data was performed using a repeatability factor of 62% that was derived from a subset of historical lakes that was visited twice in the 1970s. Only 8 of 13 lakes resurveyed had the same results on both historical visits. Our unadjusted 2006 results are a likely outcome, i.e., a 14.9% chance of finding this result, when this repeatability factor is considered, and the likelihood of no change is higher when our corrected data are considered. The possibility of double counting in the historical data further reduced the likelihood of a large decline in relative abundance. Therefore, Rusty Blackbird occurrence does not appear to have changed significantly in the past 33 yr in the Mackenzie Valley. We conclude with a qualitative discussion that supports the notion that declines in the southern parts of their range may be a large factor in the observed rates of population decline.
机译:加拿大濒危野生动植物状况委员会将“生锈黑鸟”列为“特别关注”物种,并且近几十年来种群数量急剧下降。 2006年对加拿大西北部Mackenzie谷的1970年代历史调查数据的45个地点进行了重新调查,以检查Rusty Blackbird的发生或丰度变化。我们的回顾性分析揭示了我们描述的这种比较的许多分析挑战。在过去三十年中,对调查持续时间进行校正后,出现该物种的湖泊数量似乎并未明显减少。根据圣诞节鸟类数量的数据,每年范围内下降5.1%可能导致2006年≈5个湖泊的入住率。我们估计,通过校正,该值将增加到≈26。但是,使用卡方检验的幼稚或未经调整的分析显示出明显的下降。对历史数据的模拟重采样是使用62%的重复因子进行的,该重复因子来自于1970年代两次访问过的历史湖泊子集。重新调查的13个湖泊中只有8个在两次历史性访问中都取得了相同的结果。我们的2006年未经调整的结果是可能的结果,即,当考虑到此重复性因子时,有14.9%的机会找到该结果,而考虑到校正后的数据,则无变化的可能性更高。历史数据中重复计算的可能性进一步降低了相对丰度大幅下降的可能性。因此,在过去的33年中,Mackenzie山谷的Rusty Blackbird发生率似乎没有明显变化。我们以定性讨论作为结束,该讨论支持以下观点:在该区域南部的下降可能是观察到的人口下降速度的很大因素。

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