Data from: Causes and characteristics of reverse bird migration: an analysis based on radar, radio tracking and ringing at Falsterbo, Sweden

That birds migrate in the reverse direction of the expected is a phenomenon of regular occurrence which has been observed at many sites. Here we use three different methods; tracking radar, radiotelemetry and ringing, to characterize the flights of these reverse migrants and investigate possible causes of reverse migration of nocturnally migrating passerines during autumn migration at Falsterbo peninsula, Sweden. Using these different methods we investigated both internal factors, such as age and fuel load, and external factors such as weather variables, competition and predation risk. Birds flying in the reverse direction were more likely to be lean and to be juveniles. Reverse migration was also more common with overcast skies and winds with north and east components. We did not find any effect of temperature, visibility, number of migrating sparrowhawks, or the total number of ringed birds at the site on the day of departure. We found that reverse migration is characterized by slower flight speeds (airspeed) at high altitudes and that it takes place later in the night than forward migration.鸟类沿预期方向的反向迁徙是一种普遍存在的现象，目前已在多个观测站点被观测记录。本研究采用跟踪雷达、无线电遥测与鸟类环志三种方法，对瑞典法尔斯特博半岛秋季迁徙期间的夜行性雀形目迁徙鸟类的反向迁徙飞行特征进行表征，并探究其反向迁徙的潜在成因。借助上述三种研究手段，本研究同时考察了反向迁徙的内部与外部影响因素：内部因素包括鸟类年龄与身体燃料储备，外部因素则涵盖气象变量、种间竞争与捕食风险。研究发现，发生反向迁徙的个体更大概率为体脂匮乏的幼鸟；在阴天以及带有北、东向风分量的气象条件下，反向迁徙的发生频率更高。本研究未发现气温、能见度、迁徙雀鹰的种群数量，或是当日该观测点环志鸟类的总数量对反向迁徙存在影响。结果显示，反向迁徙的鸟类在高空飞行时的空速更慢，且其夜间迁徙的启动时间晚于正向迁徙的鸟类。