Days to visit an offshore island: effect of weather conditions on arrival fuel load and potential flight range for common blackbirds Turdus merula migrating over the North Sea

Abstract

Background 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. Methods 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. Results 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. Conclusions 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.

Departure, routing and landing decisions of long-distance migratory songbirds in relation to weather

“…Such conditions might ‘force’ birds to lower their flight altitude and search for stopover or ‘rescue’ sites (e.g. [ 33 , 44 ]). Especially under poor visibility due to rain or fog, birds might collide with anthropogenic obstacles such as tall buildings or wind turbines [ 74 , 75 ].…”

“…While time-selected migrants, e.g. males during spring migration, may accept an increase in metabolic flight cost, birds that adopt other optimization criteria should show less risk-prone decisions and land rather than spend extra energy on flying under unfavourable conditions or even risk to perish at sea [ 3 , 33 ]. This especially applies to autumn migration, when birds favour a slower, less energetically costly migration compared to spring [ 8 , 10 ].…”

“…However, what actually initiates stopovers, i.e. the landing decision, remains fairly unstudied (but see [ 33 ]). Using individual tracking between stopovers, we here show that the avoidance presumption is reasonably compatible with our songbird data.…”

“…when landbirds negotiate crossing large water bodies. Since migratory songbirds are incapable of resting on water, they have to fly non-stop over water until they reach land, are vulnerable to risks resulting from deteriorating weather conditions and early exhaustion [ 29 , 32 , 33 ], and therefore should respond carefully to the current weather conditions for their migratory decisions.…”