Data from: Faster spring migration in northern wheatears is not explained by an endogenous seasonal difference in refueling rates

A widespread phenomenon in migrant birds is that they travel faster in spring than in autumn. During migration birds spend most time at stopover sites and, correspondingly, the faster spring migration is mainly explained by shorter stopovers in spring than autumn. Because a main purpose of stopovers is to replenish the fuel used in flight, a higher rate of fuel deposition (FDR) in spring is thought to explain the shorter stopovers and hence shorter total duration of migration in spring. Critical migratory processes, including the onset and extent of pre-migratory fueling, are endogenously regulated. It is therefore not unlikely that refueling at stopover sites is, at least partly, also under endogenous control. We here tested whether there is an endogenous seasonal difference in food intake and FDR, which could contribute to shorter stopovers and hence faster migration in spring. We measured daily food intake and daily FDR in two subspecies of the northern wheatear (Oenanthe oenanthe), temporarily confined at stopover under identical constant indoor conditions in spring and autumn. The two wheatear subspecies differed markedly in absolute food intake and FDR. Within subspecies, however, food intake and FDR did not differ between spring and autumn, indicating that faster spring migration in northern wheatears is not explained by an endogenously controlled seasonal difference in birds’ motivation to refuel. To further substantiate this claim, similar measurements should be taken at other locations along northern wheatears’ migration routes. Comparable experiments in other species could test the generality of our results.

候鸟普遍存在春季迁徙速度快于秋季的现象。迁徙过程中，鸟类绝大多数时间都停留于中途停歇地，因此春季迁徙更快的核心原因，被认为是春季的停歇时长较秋季更短。由于中途停歇的核心目的在于补充飞行过程中消耗的飞行燃料，学界普遍认为春季更高的燃料沉积速率（Fuel Deposition Rate, FDR）可解释春季更短的停歇时长，进而缩短整体迁徙周期。包括迁徙前储脂的启动时机与覆盖程度在内的关键迁徙过程，均受内源调控。因此，中途停歇时的加油行为至少部分也受内源调控的推测，具备合理性。本研究旨在检验：是否存在受内源调控的季节性摄食量与FDR差异，而此类差异或可促成春季更短的停歇时长与更快的迁徙速度。我们以两种北䳭（Oenanthe oenanthe）亚种为研究对象，在春季与秋季将其临时安置于统一恒定的室内模拟停歇环境中，测定其每日摄食量与每日FDR。两种北䳭亚种的绝对摄食量与FDR存在显著差异。但在同一亚种内部，春季与秋季的摄食量及FDR并无显著差异，这表明北䳭春季迁徙更快的现象，无法通过鸟类受内源调控的加油动机季节性差异来解释。为进一步验证该结论，需在北䳭迁徙路线的其他点位开展同类测定实验。在其他物种中开展同类实验，则可检验本研究结论的普适性。