Every night during spring and autumn, the mass movement of migratory birds redistributes bird abundances found on the ground during the day. However, the connection between the magnitude of nocturnal migration and the resulting change in diurnal abundance remains poorly quantified. If departures and landings at the same location are balanced throughout the night, we expect high bird turnover but little change in diurnal abundance (stream-like migration). Alternatively, migrants may move simultaneously in spatial pulses, with well-separated areas of departure and landing that cause significant changes in the abundance of birds on the ground during the day (wave-like migration). Here, we apply a flow model to data from weather surveillance radars (WSR) to quantify the daily fluxes of nocturnally migrating birds landing and departing from the ground, characterizing the movement and stopover of birds in a comprehensive synoptic scale framework. We corroborate our results with independent observations of the diurnal abundances of birds on the ground from eBird. Furthermore, we estimate the abundance turnover, defined as the proportion of birds replaced overnight. We find that seasonal bird migration chiefly resembles a stream where bird populations on the ground are continuously replaced by new individuals. Large areas show similar magnitudes of take-off and landing, coupled with relatively small distances flown by birds each night, resulting in little change in bird densities on the ground. We further show that WSR-inferred landing and take-off fluxes predict changes in eBird-derived abundance turnover rate and turnover in species composition. We find that the daily turnover rate of birds is 13% on average but can reach up to 50% on peak migration nights. Our results highlight that WSR networks can provide real-time information on rapidly changing bird distributions on the ground. The flow model applied to WSR data can be a valuable tool for real-time conservation and public engagement focused on migratory birds' daytime stopovers.

中文翻译：

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夜间鸟类迁徙导​​致日周转量高，但地面数量变化有限


春季和秋季的每个晚上，候鸟的大规模迁徙都会重新分配白天地面上发现的鸟类数量。然而，夜间迁徙的幅度与由此产生的昼间丰度变化之间的联系仍然缺乏量化。如果整个晚上在同一地点出发和降落的情况是平衡的，我们预计鸟类周转率很高，但昼夜数量变化不大（类似溪流的迁徙）。或者，候鸟可能以空间脉冲同时移动，出发和着陆区域分开，导致白天地面鸟类数量发生显着变化（波浪式迁徙）。在这里，我们将流量模型应用于天气监视雷达（WSR）的数据，以量化夜间迁徙鸟类着陆和离开地面的每日通量，在综合天气尺度框架中表征鸟类的运动和停留。我们通过 eBird 对地面鸟类昼夜丰度的独立观察证实了我们的结果。此外，我们还估计了丰度周转率，定义为隔夜更换的鸟类比例。我们发现季节性鸟类迁徙主要类似于一条溪流，地面上的鸟类种群不断被新个体取代。大面积地区的起飞和降落幅度相似，加上鸟类每晚飞行的距离相对较小，导致地面鸟类密度变化不大。我们进一步表明，WSR 推断的着陆和起飞通量可以预测 eBird 衍生的丰度周转率和物种组成周转的变化。我们发现鸟类的日周转率平均为 13%，但在迁徙高峰夜间可达 50%。 我们的结果强调，WSR 网络可以提供有关地面上快速变化的鸟类分布的实时信息。应用于 WSR 数据的流量模型可以成为关注候鸟白天停留的实时保护和公众参与的宝贵工具。