The Colorado River Delta and California's Central Valley are critical regions for many migrating North American landbirds

Migration is an important component of some species full annual cycle. California's Central Valley and the Colorado River Delta provide important riparian and wetland habitats for migrating waterbirds in the arid west of North America, but little is known about whether these locations are important at the population level to migrating landbirds. We used eBird Status and Trends abundance data to quantify the importance of the Central Valley and Colorado River Delta to landbirds by estimating the proportion of the breeding population of 112 species that use each site during migration. We found that ~17 million landbirds use the Colorado River Delta in the spring and ~14 million in the fall. Across four study regions in the Central Valley, up to ~65 million landbirds migrate through in the spring and up to ~48 million in the fall. In the spring and fall, respectively, up to 37 and up to 30 species had at least 1% of their continental population migrate through the study regions. We also quantified the spatial concentration of each species across latitudinal transects to determine the extent to which study regions were acting as migratory bottlenecks. Landbird abundances were spatially concentrated in study regions 29.4% of all migration weeks, indicating that each study region acts as a migratory bottleneck. This application of eBird data is a powerful approach to quantifying the importance of sites to migrating birds. Our results provide evidence of population-level importance of the Central Valley and Colorado River Delta for many migratory landbirds.

迁徙是部分物种完整年度生活史周期的重要组成环节。北美干旱西部的加州中央谷与科罗拉多河三角洲，为迁徙水鸟提供了关键的河岸与湿地生境，但目前学界对这两处区域是否在种群尺度上对迁徙陆栖鸟类具有重要意义仍知之甚少。我们借助eBird种群态势与趋势（Status and Trends）数据，通过估算112种迁徙陆栖鸟类的繁殖种群在途经各研究区域时的种群占比，量化了加州中央谷与科罗拉多河三角洲对陆栖鸟类的重要性。研究结果显示：春季约有1700万只陆栖鸟类途经科罗拉多河三角洲，秋季则约为1400万只；在加州中央谷的四个研究区域中，春季迁徙途经的陆栖鸟类最高可达6500万只，秋季最高约为4800万只。春秋两季分别有多达37种、30种陆栖鸟类的北美大陆种群中至少有1%会途经本次研究的区域。我们还通过沿纬度样带测算各物种种群的空间聚集程度，以此判断研究区域作为迁徙瓶颈的作用强度。在全部迁徙周中，有29.4%的时段陆栖鸟类种群在研究区域内呈现空间聚集特征，这表明各研究区域均发挥了迁徙瓶颈的作用。本研究对eBird数据的应用，为量化区域对迁徙鸟类的重要性提供了高效可靠的研究范式。本研究结果证实，加州中央谷与科罗拉多河三角洲对众多迁徙陆栖鸟类而言，具有种群尺度上的重要生态价值。