Body condition and wind support initiate the shift of migratory direction and timing of nocturnal departure in a songbird

Abstract

Summary1. An innate migration strategy guides birds through space and time. Environmental variation further modulates individual behaviour within a genetically determined frame. In particular, ecological barriers could influence departure direction and its timing. A shift in the migratory direction in response to an ecological barrier could reveal how birds adjust their individual trajectories to environmental cues and body condition. 2. Northern wheatears of the Greenland ⁄ Iceland subspecies Oenanthe oenanthe leucorhoa arrive in Western Europe en route from their West African winter range. They then undergo an endogenously controlled shift in migratory direction from north to north-west to cross a large ecological barrier, the North Atlantic. We radiotracked these songbirds departing from Helgoland, a small island in the North Sea, over an unprecedented range of their journey. 3. Here, we show that both birds' body condition and the wind conditions that they encountered influenced the departure direction significantly. Jointly high fuel loads and favourable wind conditions enabled migrants to cross large stretches of sea. Birds in good condition departed early in the night heading to the sea towards their breeding areas, while birds with low fuel loads and ⁄ or flying in poor weather conditions departed in directions leading towards nearby mainland areas during the entire night. These areas could be reached even after setting off late at night. 4. Behavioural adjustment of migratory patterns is a critical adaptation for crossing ecological barriers. The observed variation in departure direction and time in relation to fuel load and wind revealed that these birds have an innate ability to respond by jointly incorporating internal information (body condition) and external information (wind support).

Fat, weather, and date affect migratory songbirds’ departure decisions, routes, and time it takes to cross the Gulf of Mexico

“…Songbirds departing from coastal Alabama were able to assess departure conditions to take advantage of favorable circumstances for crossing the Gulf safely and quickly. Previous songbird data in support of this hypothesis along the edges of large geographic features have only considered either departure or arrival behavior (20,31,40,41,47), and the proximate cues and decision rules used to identify favorable conditions have been unclear (40). By detecting songbirds after negotiating ∼1,000 km of open water, we identified conditions affecting both departure and arrival, providing strong empirical support for the hypothesis that conditions that affect departure also influence crossing behaviors.…”

“…The role of these factors is likely species specific. Energetic reserves (primarily fat but also protein) contribute to reducing the risks of migrating where foraging options are scarce and/or energetic demands are elevated (31,32). Age may affect migratory decisions because older individuals might use previous experience to optimize their travel and assess and manage risk (33,34).…”

“…Studies of migratory departure decisions of small birds along edges of large geographic features, including the GOM, have contributed substantially to our understanding of how they assess and manage risk at the onset of flight (20,28,31,41), but the consequences of these decisions on migratory routes, crossing times, and arrival on the other side are unknown. Light-geolocator studies have advanced our understanding of general migration routes of small birds in relation to geographic features and overall rates of migration (42), but the low spatial and temporal precision of these studies do not permit analyses of how behaviors are informed by dynamic intrinsic and extrinsic variables at fine scales.…”

Size and accumulation of fuel reserves at stopover predict nocturnal restlessness in a migratory bird

“…In May 2013, 42 migrating northern wheatears were caught using mealwormbaited spring traps and subsequently caged for three days. Upon capture, CE measured wing length (maximum chord) to the nearest 0.5 mm, and body mass to the nearest 0.1 g. Wing length was used to calculate lean body mass, employing a linear regression based on 220 'lean' northern wheatears caught on Helgoland in previous years: lean body mass (g) ¼ 0.29 g mm 21 Â wing length (mm) -6.85 g (linear regression: n ¼ 220, F 1,218 ¼ 95.07, adj-R 2 ¼ 0.30, p , 0.0001, after [19]). Fuel load was calculated as: fuel load ¼ (body mass (g) -lean body mass (g))/lean body mass (g).…”

Acoustic monitoring of migratory birds over western Lake Erie: avian responses to barriers and the importance of islands

“…Numerous factors might lead birds to select circuitous routes rather than cross over ecological barriers (Alerstam 2001), including their own body condition and external information such as weather conditions (Schmaljohann and Naef-Daenzer 2011). The increased risk of crossing an ecological barrier may be offset by the time or energy saved in using the shortest migratory route (Bruderer 2001).…”