Interspecific Comparison of the Performance of Soaring Migrants in Relation to Morphology, Meteorological Conditions and Migration Strategies

BackgroundPerformance of migrating birds can be affected by a number of intrinsic and extrinsic factors like morphology, meteorological conditions and migration strategies. We compared travel speeds of four raptor species during their crossing of the Sahara desert. Focusing the analyses on this region allows us to compare different species under equivalent conditions in order to disentangle which factors affect migratory performance. Methodology/Principal FindingWe tracked raptors using GPS satellite transmitters from Sweden, Spain and Italy, and evaluated their migratory performance at both an hourly and a daily scale. Hourly data (flight speed and altitude for intervals of two hours) were analyzed in relation to time of day, species and season, and daily data (distance between roosting sites) in relation to species, season, day length and tailwind support. Conclusions/SignificanceDespite a clear variation in morphology, interspecific differences were generally very small, and did only arise in spring, with long-distance migrants (>5000 km: osprey and Western marsh-harrier) being faster than species that migrate shorter distances (Egyptian vulture and short-toed eagle). Our results suggest that the most important factor explaining hourly variation in flight speed is time of day, while at a daily scale, tailwind support is the most important factor explaining variation in daily distance, raising new questions about the consequences of possible future changes in worldwide wind patterns.研究背景：迁徙鸟类的迁飞表现会受到诸多内在与外在因素的影响，诸如形态特征、气象条件以及迁徙策略等。本研究针对四种猛禽在穿越撒哈拉沙漠过程中的飞行速度开展对比分析。将分析区域限定于该沙漠，可使不同物种在同等环境条件下进行比较，从而厘清哪些因素会对迁飞表现产生影响。 研究方法与主要结果：我们通过部署于瑞典、西班牙与意大利的GPS卫星追踪器（GPS satellite transmitters）对猛禽进行追踪，并分别从小时尺度与日尺度两个维度评估其迁飞表现。针对小时尺度数据（每两小时记录一次的飞行速度与飞行高度），我们结合当日时段、物种与季节因素开展分析；针对日尺度数据（栖息位点间的日飞行距离），则结合物种、季节、昼长与顺风助力（tailwind support）因素展开分析。 结论与意义：尽管不同猛禽的形态特征存在显著差异，但总体而言其种间差异极小，且仅在春季呈现出显著差异：长距离迁徙物种（迁徙距离＞5000千米：鹗（osprey）与西方泽鹞（Western marsh-harrier））的飞行速度快于短距离迁徙物种（埃及秃鹫（Egyptian vulture）与短趾雕（short-toed eagle））。本研究结果显示，解释飞行速度小时尺度变化的最关键因素为当日时段；而在日尺度上，解释日飞行距离变化的最关键因素为顺风助力。该研究结果为全球风场未来可能发生变化所带来的影响提出了全新的研究议题。