Kelsey Natalie A, Hüppop Ommo, Bairlein Franz
Institute of Avian Research 'Vogelwarte Helgoland', An der Vogelwarte 21, 26386, Wilhelmshaven, Germany.
Max Planck Institute of Animal Behavior, Am Obstberg 1, 78315, Radolfzell, Germany.
Mov Ecol. 2021 Oct 21;9(1):53. doi: 10.1186/s40462-021-00290-6.
Crossing open water instead of following the coast(line) is one way for landbirds to continue migration. However, depending on prevailing weather and the birds' physiological conditions, it is also a risky choice. To date, the question remains as to which interplay between environmental and physiological conditions force landbirds to stop on remote islands. We hypothesise that unfavourable winds affect lean birds with low energy resources, while poor visibility affects all birds regardless of their fuel loads.
To test this hypothesis, we caught 1312 common blackbirds Turdus merula stopping over on Helgoland during autumn and spring migration. Arrival fuel load was measured using quantitative magnetic resonance technology. Weather parameters (wind and relative humidity as a proxy for visibility) were interpolated for the night before arrival. Further, we calculated whether caught individuals would have successfully crossed the North Sea instead of landing on Helgoland, depending on wind conditions.
Both wind and relative humidity the night before arrival were correlated with arrival fuel load. After nights with strong headwinds, birds caught the following day were mostly lean, most of which would not have managed to cross the sea if they had not stopped on Helgoland. In contrast, fat birds that could have successfully travelled on were caught mainly after nights with high relative humidity (≥ 80%). Furthermore, the rate of presumably successful flights was lower due to wind: although only 9% of all blackbirds captured on Helgoland had insufficient fuel loads to allow safe onward migration in still air, real wind conditions would have prevented 30% of birds from successfully crossing the sea during autumn and 21% during spring migration.
We were able to decipher how physiological condition, wind and relative humidity partially force blackbirds to stop on a remote island. Adverse winds tend to affect lean birds with low energy resources, while poor visibility can affect blackbirds, regardless of whether the arrival fuel load was sufficient for onward flight. Our findings will help to understand different migratory strategies and explain further questions like migration timing.
陆地鸟类继续迁徙的一种方式是穿越开阔水域而非沿着海岸线飞行。然而，根据当时的天气和鸟类的生理状况，这也是一个冒险的选择。迄今为止，关于环境和生理条件之间的何种相互作用迫使陆地鸟类在偏远岛屿停留的问题仍然存在。我们推测，不利的风会影响能量资源匮乏的瘦鸟，而能见度差则会影响所有鸟类，无论它们的能量储备如何。
为了验证这一假设，我们在秋季和春季迁徙期间捕获了1312只在黑尔戈兰岛停留的欧歌鸫。使用定量磁共振技术测量到达时的能量储备。对到达前一晚的天气参数（风以及作为能见度替代指标的相对湿度）进行内插。此外，我们根据风况计算了捕获的个体是否本可以成功穿越北海而不是降落在黑尔戈兰岛。
到达前一晚的风和相对湿度均与到达时的能量储备相关。在经历强逆风的夜晚之后，次日捕获的鸟类大多很瘦，如果它们没有在黑尔戈兰岛停留，其中大多数将无法成功跨海。相比之下，本可以成功继续飞行的肥鸟主要是在相对湿度较高（≥80%）的夜晚之后被捕获的。此外，由于风的原因，推测的成功飞行率较低：尽管在黑尔戈兰岛捕获的所有欧歌鸫中只有9%的能量储备不足以在静风条件下安全继续迁徙，但实际风况会导致30%的鸟类在秋季和21%的鸟类在春季迁徙期间无法成功跨海。
我们能够解读生理状况、风和相对湿度如何部分地迫使欧歌鸫在偏远岛屿停留。逆风往往会影响能量资源匮乏的瘦鸟，而能见度差会影响欧歌鸫，无论到达时的能量储备是否足以继续飞行。我们的研究结果将有助于理解不同的迁徙策略，并解释诸如迁徙时间等进一步的问题。
