Data_Sheet_1_Groping in the Fog: Soaring Migrants Exhibit Wider Scatter in Flight Directions and Respond Differently to Wind Under Low Visibility Conditions.DOCX

Atmospheric conditions are known to affect flight propensity, behaviour during flight, and migration route in birds. Yet, the effects of fog have only rarely been studied although they could disrupt orientation and hamper flight. Fog could limit the visibility of migrating birds such that they might not be able to detect landmarks that guide them during their journey. Soaring migrants modulate their flight speed and direction in relation to the wind vector to optimise the cost of transport. Consequently, landmark-based orientation, as well as adjustments of flight speed and direction in relation to wind conditions, could be jeopardised when flying in fog. Using a radar system operated in a migration bottleneck (Strait of Messina, Italy), we studied the behaviour of soaring birds under variable wind and fog conditions over two consecutive springs (2016 and 2017), discovering that migrating birds exhibited a wider scatter of flight directions and responded differently to wind under fog conditions. Birds flying through fog deviated more from the mean migration direction and increased their speed with increasing crosswinds. In addition, airspeed and groundspeed increased in the direction of the crosswind, causing the individuals to drift laterally. Our findings represent the first quantitative empirical evidence of flight behaviour changes when birds migrate through fog and explain why low visibility conditions could risk their migration journey.

已有研究表明，大气条件会影响鸟类的飞行意愿、飞行行为以及迁徙路线。然而，尽管雾可能干扰鸟类定向、阻碍其飞行，但相关研究却鲜有涉及雾的影响。雾会降低迁徙鸟类的能见度，使其无法识别旅途中用于导航的地标。翱翔型迁徙鸟类会依据风矢量调节飞行速度与方向，以优化飞行能耗。因此，当鸟类在雾中飞行时，基于地标的定向能力，以及依据风力条件调节飞行速度与方向的行为，均可能受到损害。本研究依托部署于意大利墨西拿海峡这一鸟类迁徙瓶颈带的雷达系统，在2016年与2017年连续两个春季期间，对不同风力与雾况下的翱翔型迁徙鸟类行为开展观测，发现雾天条件下迁徙鸟类的飞行方向离散度更广，且对风力的响应模式存在显著差异。穿越雾区的鸟类偏离平均迁徙方向的程度更高，且随着侧风强度增大，其飞行速度会随之提升。此外，鸟类的空速与地速会沿侧风方向出现增幅，导致个体发生侧向漂移。本研究的发现首次提供了鸟类穿越雾区时飞行行为改变的定量实证证据，并解释了低能见度条件为何会对鸟类迁徙行程构成风险。