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Zusammenfassung der Projektergebnisse
Billions of songbirds perform seasonal migrations between their breeding areas and wintering grounds. During migration, many songbird migrants usually follow a stop-and-go strategy with alternating cycles of energy accumulation and nocturnal migratory movements towards their destinations. Most of them travel without social guidance. To arrive at the correct place and time, their innate migration program therefore defines birds’ spatiotemporal and physiological organisation of migration and their typical reactions to environmental cues encountered during migration. Although the basic circannual and circadian rhythms of songbirds, their adaptation to migration, and the factors influencing the birds’ dayto-day departure decision are reasonably well studied, we do not understand how birds time their departures within the night. Yet, these departure decisions are crucial, because the nocturnal departure time defines the potential flight duration of the migratory night. The distances covered during the nocturnal migratory flights in the course of migration in turn directly affect the overall speed of migration. To better understand the factors influencing the arrival of the birds in the breeding/wintering areas, we investigated here the mechanisms that control nocturnal departure timing. For this, we outlined a conceptual framework for explaining variation in this trait and suggested that the endogenously controlled schedule of nocturnal departures is modified by intrinsic and extrinsic factors. Controlling for and manipulating both type of factors in wild, free-flying songbirds is notoriously difficult. To address this challenge, we made use of the fact that wild migratory birds kept in cages during migration show nocturnal activity, called migratory restlessness. We could show that the start of nocturnal migratory restlessness of caged songbirds was significantly positively related to their radiotracked nocturnal departure time the following night. This provides the essential basis for experimental study of the start of nocturnal migratory restlessness and with that nocturnal departure time. Identifying the nocturnal departure timing of free-flying songbirds is another challenge, because take-offs from a stopover site must be distinguished from nocturnal exploratory flights and landscape movements, which are both a completely different behavioural response in comparison to the bird’s decision of departing from a stopover site and thus actually resuming migration. To do so, we installed an array of digital automated radio-tracking stations with 12 horizontally mounted antennas being aligned radially at intervals of c. 30° on Helgoland. This set-up allowed us to jointly combine cage experiments in which intrinsic and extrinsic conditions could be controlled for together with studies of free-flying birds. Based on this, we demonstrated that remaining migration distance, upcoming nature of migratory route, progress of the migration season, energy stores, and tailwind conditions significantly negatively affect the bird’s decision to start is nocturnal migratory behaviour, either restlessness or departure. Could cover had a positive effect. We experimentally decoupled the length of the night and the time within season, so that birds were exposed to nights of either natural or shortened length. We found that birds exposed to shortened nights showed slightly earlier nocturnal departures than those exposed to the natural length of the night, but not difference in their start of nocturnal migratory restlessness. Our studies suggest that the innate migration program of birds facilitates a complex decision-making process. In this process, bird’s responses to different intrinsic and extrinsic factors are interdependent. This means that bird’s evaluation of one factor may change if the condition of another factor altered before. Such flexible adjustments of the behavioural response to intrinsic and extrinsic factors help to explain discrepant results on their effect on nocturnal departure timing in former studies. Further, it emphasizes the importance of interpreting data on birds’ nocturnal departure timing in the respective ecological context, i.e., the corresponding migration season, the entire migration route, and the currently encountered landscapes and habitats.
Projektbezogene Publikationen (Auswahl)
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(2018) Does the length of the night affect the timing of nocturnal departures in a migratory songbird?. Animal Behaviour 141 183–194
Müller, Florian; Rüppel, Georg; Schmaljohann, Heiko
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2015. Start of nocturnal migratory restlessness in captive birds predicts nocturnal departure time in free-flying birds. Behavioral Ecology and Sociobiology 69: 909-914
Schmaljohann, H., Kämpfer, S., Fritzsch, A., Kima, R. & Eikenaar, C.
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2016. Fuel accumulation advances nocturnal departure: A migratory restlessness study on northern wheatears at stopover. Animal Behavior 117: 9-14
Eikenaar, C. , Müller, F., Kämpfer, S. & Schmaljohann, H.
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2016. Towards a conceptual framework for explaining variation in the nocturnal departure time of songbird migrants. Movement Ecology 4: 24
Müller, F., Taylor, P.D., Sjöberg, S., Muheim, R., Tsvey, A., Mackenzie, S.A. & Schmaljohann, H.
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2017. Corticosterone and timing of migratory departure in a songbird. Proceeding of the Royal Society of London B 284: 20162300
Eikenaar, C., Müller, F., Leutgeb, C., Hessler, S., Lebus, K., Taylor, P.D. & Schmaljohann, H.
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2018. Automatische Verfolgung kleiner Vögel – Der technische Fortschritt hält Einzug in die Vogelzugforschung über der Nordsee. Nachrichten des Marschenrates 55: 57-62
Schmaljohann, H., Brust, V. & Hüppop, O.
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2018. Nocturnal departure timing in songbirds facing distinct migratory challenges. Journal of Animal Ecology
Müller, F., Eikenaar, C., Crysler, Z.J., Taylor, P.D. & Schmaljohann, H.
