Abstract
Migrating birds spend most of their time at stopover sites where they replenish the fuel used during flight, termed refueling. The overall time of migration thus largely depends on the duration of stopovers, and factors shaping stopover duration therefore are of interest. A handful of field studies have shown that the likelihood of departure from stopover sites increases with poor feeding conditions. However, food availability and stopover duration are generally difficult to quantify accurately in the field. Results of fasting-refueling experiments on captive birds using migratory restlessness (Zugunruhe) as a proxy for departure likelihood are mixed. Although Zugunruhe usually decreased with refueling, fasting often failed to increase Zugunruhe. In addition, some experiments lacked randomization. In a fasting-refueling experiment on Northern Wheatears (Oenanthe oenanthe), using birds as their own control in a randomized fashion, we found that fasting increased Zugunruhe, whereas refueling decreased Zugunruhe. These results show that the motivation to migrate, quantified by Zugunruhe, is affected by changes in food availability. Furthermore, Zugunruhe during refueling did not depend on fuel reserves left after fasting, but tended to decrease with the amount of fuel lost during fasting. We discuss why extent of fuel loss may be a better predictor of stopover duration than fuel reserves.
Zusammenfassung
Nahrungsverfügbarkeit und Körpermassenverlust bedingen Zugunruhe
Zugvögel verbringen in Rastgebieten während des Zuges die meiste Zeit damit, im Flug vorher verbrauchte Energiereserven wieder aufzufüllen („aufzutanken“). Deshalb hängt die Zuggeschwindigkeit insbesondere von der Dauer der Rastaufenhalte ab, weshalb Faktoren, die diese bestimmen, von großem Interesse sind. Eine Handvoll von Arbeiten hat gezeigt, dass die Wahrscheinlichkeit, ein Rastgebiet wieder zu verlassen, ansteigt, wenn die Nahrungsbedingungen schlecht sind. Allerdings ist die Erfassung von Nahrungsverfügbarkeit und Rastdauer im Freiland ganz allgemein schwierig zu quantifizieren. Sog. „Fasten-Auftanken“-Experimente („fasting-refuelling“) an gekäfigten Vögeln, die Zugunruhe als Merkmal für Abflugwahrscheinlichkeit nutzen, ergaben unterschiedliche Ergebnisse. Auch wenn die Zugunruhe üblicherweise abnahm, sobald die Vögel nach Fasten wieder Futter bekamen, erhöhte das Fasten selbst die Zugunruhe meist nicht. Zudem mangelt es manchen dieser Studien an fehlender Randomisierung. In einem „fasting-refuelling“Experiment mit Steinschmätzern (Oenanthe oenanthe), in dem wir den einzelnen Vogel randomisiert als seine eigene Kontrolle nutzten, fanden wir, dass während des Fasten die Zugunruhe zunahm, während sie bei erneuter Massenzunahme („refuelling“) abnahm. Dies zeigt, dass die Motivation zu ziehen, ausgedrückt über die gemessene Zugunruhe, von der Nahrungsverfügbarkeit bestimmt ist. Weiterhin war die Zugunruhe während des „refuelling“nicht von der nach dem Fasten noch restlichen Energiemenge abhängig. Vielmehr nahm sie mit dem Ausmaß des Energieverlustes während des Fastens ab. In der Diskussion erörtern wir, warum der Verlust an „Treibstoff“eine bessere Vorhersage der Rastdauer erlaubt als der Energievorrat selbst.
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Acknowledgments
We thank Ulrich Meyer and Adolf Völk for their assistance in the experiment, Marc Bulte for help with the Zugunruhe equipment, and two anonymous reviewers for their constructive comments.
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Communicated by L. Fusani.
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Eikenaar, C., Bairlein, F. Food availability and fuel loss predict Zugunruhe. J Ornithol 155, 65–70 (2014). https://doi.org/10.1007/s10336-013-0987-7
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DOI: https://doi.org/10.1007/s10336-013-0987-7