![]() ![]() 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. In contrast, fat birds that could have successfully travelled on were caught mainly after nights with high relative humidity (≥ 80%). 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. Resultsīoth wind and relative humidity the night before arrival were correlated with arrival fuel load. Further, we calculated whether caught individuals would have successfully crossed the North Sea instead of landing on Helgoland, depending on wind conditions. Weather parameters (wind and relative humidity as a proxy for visibility) were interpolated for the night before arrival. Arrival fuel load was measured using quantitative magnetic resonance technology. To test this hypothesis, we caught 1312 common blackbirds Turdus merula stopping over on Helgoland during autumn and spring migration. We hypothesise that unfavourable winds affect lean birds with low energy resources, while poor visibility affects all birds regardless of their fuel loads. To date, the question remains as to which interplay between environmental and physiological conditions force landbirds to stop on remote islands. However, depending on prevailing weather and the birds’ physiological conditions, it is also a risky choice. Crossing open water instead of following the coast(line) is one way for landbirds to continue migration.
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