Data from: A characterization of autumn nocturnal migration detected by weather surveillance radars in the northeastern US

Billions of birds migrate at night over North America each year. However, few studies have described the phenology of these movements, such as magnitudes, directions, and speeds, for more than one migration season and at regional scales. In this study, we characterize density, direction, and speed of nocturnally migrating birds using data from 13 weather surveillance radars in the autumns of 2010 and 2011 in the northeastern US. After screening radar data to remove precipitation, we applied a recently developed algorithm for characterizing velocity profiles with previously developed methods to document bird migration. Many hourly radar scans contained wind-borne "contamination," and these scans also exhibited generally low overall reflectivities. Hourly scans dominated by birds showed nightly and seasonal patterns that differed markedly from those of low reflectivity scans. Bird migration occurred during many nights, but a smaller number of nights with large movements of birds defined regional nocturnal migration. Densities varied by date, time, and location but peaked in the second and third deciles of night during the autumn period when the most birds were migrating. Migration track (the direction to which birds moved) shifted within nights from south southwesterly to southwesterly during the seasonal migration peaks; this shift was not consistent with a similar shift in wind direction. Migration speeds varied within nights, although not closely with wind speed. Airspeeds increased during the night; groundspeeds were highest between the second and third deciles of night, when the greatest density of birds was migrating. Airspeeds and groundspeeds increased during the fall season, although groundspeeds fluctuated considerably with prevailing winds. Significant positive correlations characterized relationships among bird densities at southern coastal radar stations and northern inland radar stations. The quantitative descriptions of broad-scale nocturnal migration patterns presented here will be essential for biological and conservation applications. These descriptions help to define migration phenology in time and space, fill knowledge gaps in avian annual cycles, and are useful for monitoring long-term population trends of migrants. Furthermore, these descriptions will aid in assessing potential risks to migrants, particularly from structures with which birds collide and artificial lighting that disorients migrants.

每年有数十亿只鸟类在北美上空开展夜间迁徙。然而，目前鲜有研究能够在区域尺度上、跨越多个迁徙季，系统刻画此类迁徙活动的物候特征——涵盖迁徙规模、行进方向与飞行速度等核心参数。本研究依托2010年与2011年秋季美国东北部13部天气监视雷达（weather surveillance radars）的观测数据，解析了夜间迁徙鸟类的种群密度、飞行方向与飞行速度。在对雷达数据开展降水回波剔除预处理后，我们结合已有的鸟类迁徙探测方法，应用了一套新近开发的速度剖面特征提取算法。大量逐小时雷达扫描数据存在风致‘杂波污染’，且整体反射率普遍偏低；而以鸟类回波为主的逐小时扫描数据，其夜间与季节尺度的变化模式与低反射率扫描结果存在显著差异。多数夜间均有鸟类迁徙活动，但仅少数夜间出现大规模集群迁徙，构成区域夜间迁徙的核心时段。鸟类种群密度随观测日期、时段与空间位置发生动态变化，在秋季迁徙盛期，其密度在夜间进程的第二至第三十分位区间达到峰值，此时迁徙鸟类数量最多。在季节性迁徙盛期，鸟类的飞行轨迹（即鸟类的行进方向）在夜间从南西南向逐渐转为西南向，这一变化趋势与风向的类似偏移并不一致。夜间飞行速度存在日内波动，但与风速并无显著关联。空速（airspeeds）在夜间逐渐提升；地速（groundspeeds）则在夜间进程的第二至第三十分位区间达到最高，此时鸟类迁徙密度也最大。空速与地速在秋季迁徙季整体呈升高趋势，但地速受盛行风影响出现明显波动。美国南部沿海雷达站与北部内陆雷达站所观测到的鸟类密度间存在显著正相关关系。本研究给出的大范围夜间迁徙模式定量描述，对鸟类学研究与物种保护实践均具有重要价值：此类描述明确了迁徙物候的时空分布特征，填补了鸟类年度生活史研究中的知识空白，可用于监测迁徙鸟类的长期种群趋势；此外，还有助于评估迁徙鸟类面临的潜在风险，尤其是鸟类撞击人工构筑物以及人工照明误导迁徙鸟类所带来的风险。