Data from: Directed flight and optimal airspeeds: homeward-bound gulls react flexibly to wind yet fly slower than predicted

Birds in flight are proposed to adjust their body orientation (heading) and airspeed to wind conditions adaptively according to time and energy constraints. Airspeeds in goal-directed flight are predicted to approach or exceed maximum-range airspeeds, which minimize transport costs (energy expenditure per unit distance) and should increase in headwinds and crosswinds. Diagnosis of airspeed adjustment is however obscured by uncertainty regarding birds' goal-directions, transport costs, interrelations with orientation strategy and the attainability of predicted behaviour. To address these issues, we tested whether gulls minimized transport costs through adjustment of airspeed and heading to wind conditions during extended inbound flight over water (180–360 km) to their breeding colony, and introduce a methodology to assess transport (energy) efficiency given wind conditions. Airspeeds, heading, flight mode and energy expenditure were estimated using GPS tracking, accelerometer and wind data. Predicted flight was determined by simulating each trip according to maximum-range airspeeds and various orientation strategies. Gulls employed primarily flapping flight (93%), and negotiated crosswinds flexibly to exploit both high altitude tailwinds and coastal soaring opportunities. We demonstrate that predicted airspeeds in heavy crosswinds depend strongly on orientation strategy and presumed preferred direction. Measured airspeeds increased with headwind and crosswind similarly to maximum-range airspeeds based on full compensation for wind drift, yet remained ∼ 30% lower than predicted by all strategies, resulting in slower and 30–35% costlier flight. Interestingly, more energy could be saved through adjustment of airspeed (median 40%) than via orientation strategy (median 4%). Therefore, despite remarkably flexible reaction to wind at sea, these gulls evidently minimized neither time nor energy expenditure. However, airspeeds were possibly over-predicted by current aerodynamic models. This study emphasizes the importance of accounting for orientation strategy when assessing airspeed adjustments to wind and indicates that either the cost or adaptive ‘currency’ of extended flight among gulls may require revision.已有研究提出，飞行中的鸟类可根据时间与能量约束，自适应调整身体朝向（heading，航向）与空速（airspeed）以适配风况。在有目标导向的飞行中，空速预计会接近或超过最大航程空速（maximum-range airspeeds）——该空速可最小化运输成本（transport costs，即单位距离的能量消耗），且在逆风与侧风中会有所提升。然而，受限于鸟类目标方向、运输成本、与朝向策略的关联，以及预测行为可实现性等方面的不确定性，针对空速调整的相关分析难以推进。为解决上述问题，本研究针对鸥类跨海面的长距离归巢飞行（180–360 km）——其目的地为繁殖群栖息地——展开实验，验证鸥类是否会通过调整空速与朝向以适配风况，从而最小化运输成本，并提出了一种在给定风况下评估运输（能量）效率的方法。研究人员通过GPS追踪（GPS tracking）、加速度计（accelerometer）数据与风场数据，估算了空速、航向、飞行模式与能量消耗。通过基于最大航程空速与多种朝向策略模拟每一次飞行，得到预测飞行状态。鸥类主要采用振翅飞行（占比93%），并能灵活应对侧风，既可利用高空顺风，也可借助海岸翱翔的机会。本研究表明，强侧风环境下的预测空速在很大程度上取决于朝向策略与假定的偏好方向。实测空速随逆风与侧风的变化趋势，与完全补偿风漂移后的最大航程空速预测结果相似，但仍比所有策略的预测值低约30%，导致飞行速度更慢，且飞行成本增加30–35%。值得注意的是，通过调整空速可节省的能量（中位数为40%）远多于通过调整朝向策略所能节省的能量（中位数为4%）。因此，尽管这些鸥类对海上风况展现出极强的灵活适应性，但它们显然既没有最小化飞行时间，也没有最小化能量消耗。不过，当前的空气动力学模型可能高估了鸥类的空速。本研究强调，在评估鸟类针对风况的空速调整行为时，考虑朝向策略的重要性，并指出鸥类长距离飞行的成本或自适应的“货币”（即优化目标）可能需要重新审视。