Regional Distribution Shifts Help Explain Local Changes in Wintering Raptor Abundance: Implications for Interpreting Population Trends

Studies of multiple taxa across broad-scales suggest that species distributions are shifting poleward in response to global climate change. Recognizing the influence of distribution shifts on population indices will be an important part of interpreting trends within management units because current practice often assumes that changes in local populations reflect local habitat conditions. However, the individual- and population-level processes that drive distribution shifts may occur across a large, regional scale and have little to do with the habitats within the management unit. We examined the latitudinal center of abundance for the winter distributions of six western North America raptor species using Christmas Bird Counts from 1975–2011. Also, we considered whether population indices within western North America Bird Conservation Regions (BCRs) were explained by distribution shifts. All six raptors had significant poleward shifts in their wintering distributions over time. Rough-legged Hawks (Buteo lagopus) and Golden Eagles (Aquila chrysaetos) showed the fastest rate of change, with 8.41 km yr−1 and 7.74 km yr−1 shifts, respectively. Raptors may be particularly responsive to warming winters because of variable migration tendencies, intraspecific competition for nesting sites that drives males to winter farther north, or both. Overall, 40% of BCR population trend models were improved by incorporating information about wintering distributions; however, support for the effect of distribution on BCR indices varied by species with Rough-legged Hawks showing the most evidence. These results emphasize the importance of understanding how regional distribution shifts influence local-scale population indices. If global climate change is altering distribution patterns, then trends within some management units may not reflect changes in local habitat conditions. The methods used to monitor and manage bird populations within local BCRs will fundamentally change as species experience changes in distribution in response to climate change.

跨大尺度范围的多类群研究表明，物种分布正朝着极地方向偏移以响应全球气候变化。认识到分布偏移对种群指数的影响，是解读管理单元内种群趋势的重要环节，因为当前的研究实践通常假设本地种群的变化反映了当地栖息地的状况。然而，驱动分布偏移的个体及种群级过程，可能发生在广阔的区域尺度上，与管理单元内的栖息地并无多大关联。本研究利用1975-2011年的圣诞鸟类计数（Christmas Bird Count）数据，分析了北美西部6种猛禽冬季分布的纬度丰度中心；同时探究了北美西部鸟类保护区域（Bird Conservation Region，BCR）内的种群指数是否可由分布偏移来解释。所有6种猛禽的冬季越冬分布均随时间发生了显著的向极地偏移。毛脚鵟（Buteo lagopus）与金雕（Aquila chrysaetos）的变化速率最快，偏移速率分别为8.41 km·yr⁻¹和7.74 km·yr⁻¹。由于迁徙倾向存在差异、巢址种内竞争驱使雄性个体向更北的区域越冬，或是两者共同作用，猛禽可能对冬季变暖尤为敏感。总体而言，通过纳入冬季分布相关信息，40%的BCR种群趋势模型得到了优化；但不同物种的分布对BCR种群指数的影响效应存在差异，其中毛脚鵟的相关证据最为充分。这些研究结果强调了理解区域分布偏移如何影响局域尺度种群指数的重要性。倘若全球气候变化正在改变物种分布格局，那么部分管理单元内的种群趋势可能无法反映当地栖息地的变化情况。随着物种为响应气候变化而发生分布偏移，用于局域BCR内鸟类种群监测与管理的方法将发生根本性变革。