Abstract
In western Europe, the coasts of Vendée and Charente-Maritime on the central Atlantic shoreline of France constitute the most southern wintering site for the Red Knot Calidris canutus islandica, welcoming around 37,000 individuals each winter, and also represent a key stopover area for C. c. canutus (up to 60,000 individuals) between Africa and the Wadden Sea in spring. Nevertheless, the origin of the birds in France arriving in the autumn is unclear, considering that the first islandica start to appear for wintering while canutus could use the same sites as a stopover on their migration route to Africa. We used biometric data from birds caught between 1983 and 2008 to assess the origin of Red Knot staging and wintering in France, and we also investigated the age structure of Red Knot groups during three distinctive periods of their biological cycle: autumn migration, wintering and spring migration. This first assessment was completed by data analysis of ring recoveries over a period of 44 years and isotopic ratio signatures of feathers. The obtained results were in conformity with the general patterns of distribution throughout the year and the timing of migration of both subspecies in Europe as described in previous reviews. Nevertheless, this study highlights two unknown features for both subspecies in this part of their European area distributions. First, a very large predominance of juveniles was recorded at the expense of an extreme rarity of adults in autumn and early winter. We prove the presence of canutus-knots among these juveniles, even with staging later in the autumn and the possibility of wintering there for some of them. Although the Atlantic coast of France welcomes only 9% of the population of islandica-knots wintering in Europe, this network of estuarine bays could represent a crucial strategic area for juveniles. The place could constitute a liberated area for islandica juveniles coming later than adults already settled in northern sites. It offers the possibility for canutus to migrate south on distance-limited stages in order to experiment with the flyway and refuel to join the traditional African wintering grounds.
Zusammenfassung
In Westeuropa stellen die Küsten von Vendée und Charente-Maritime an der zentralatlantischen Küste von Frankreich das südlichste Überwinterungsgebiet für die Knutt-Unterart Calidris canutus islandica dar, wo sich jeden Winter etwa 37.000 Individuen versammeln. Sie sind außerdem ein wichtiges Rastgebiet für die Unterart C. c. canutus (bis zu 60.000 Individuen) auf ihrem Zug zwischen Afrika und dem Wattenmeer im Frühjahr. Dennoch ist die Herkunft der Vögel, die im Herbst in Frankreich eintreffen, unklar, wenn man bedenkt, dass die ersten islandica-Knutts zum Überwintern ankommen, während canutus-Knutts dieselben Gebiete als Rastplätze auf ihrem Zug nach Afrika nutzen könnten. Wir haben biometrische Daten von zwischen 1983 und 2008 gefangenen Vögeln verwendet, um die Herkunft in Frankreich rastender und überwinternder Knutts festzustellen. Auch haben wir die Altersstruktur von Knuttgruppen während dreier unterschiedlicher Phasen in ihrem biologischen Zyklus untersucht: Herbstzug, Überwintern und Frühjahrszug. Diese erste Einschätzung wurde durch eine Datenanalyse von Ringwiederfunden über einen Zeitraum von 44 Jahren und Isotopensignaturen von Federn ergänzt. Die gewonnenen Ergebnisse stimmten mit den allgemeinen Verbreitungsmustern über das Jahr und der zeitlichen Koordinierung des Zuges der beiden Unterarten in Europa überein, wie sie in früheren Übersichtsartikeln beschrieben worden waren. Dennoch hebt diese Studie zwei bislang unbekannte Besonderheiten für beide Unterarten in diesem Teil ihres europäischen Verbreitungsgebietes hervor. Erstens wurde im Herbst und Frühwinter ein starkes Überwiegen juveniler Tiere auf Kosten adulter Vögel, die extrem selten waren, festgestellt. Zudem erbringen wir den Nachweis für die Anwesenheit von canutus-Knutts unter diesen juvenilen Tieren, auch solchen, die später im Herbst rasten und für welche die Möglichkeit zur dortigen Überwinterung besteht. Obwohl die französische Atlantikküste lediglich 9% der in Europa überwinternden islandica-Knutts beherbergt, könnte dieses Netzwerk ästuarer Buchten ein für juvenile Tiere äußerst wichtiges Gebiet sein. Es könnte ein freies Areal für Juvenile, die später kommen als Altvögel, welche sich bereits in den nördlichen Gebieten niedergelassen haben, darstellen. Es bietet die Möglichkeit für canutus-Knutts, auf Etappen begrenzter Entfernung nach Süden zu ziehen, um mit den Zugwegen zu experimentieren und aufzutanken, um die traditionellen afrikanischen Überwinterungsgebiete zu erreichen.
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Acknowledgments
The catches on study sites were only possible with the precious help of numerous people from The National Nature Reserves and volunteers. We would like to thank the staff of the Nature Reserves of Marais d’Yves, Lileau des Niges, Baie de l’Aiguillon and Moëze-Oléron. We particularly thank Julien Gonin, Pierre Rousseaux, Fréderic Corre and Nicolas Boileau.
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Communicated by F. Bairlein.
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Bocher, P., Quaintenne, G., Robin, F. et al. Origins and age structure of Red Knot Calidris canutus staging and wintering on the Atlantic coast of France. J Ornithol 153, 103–114 (2012). https://doi.org/10.1007/s10336-011-0702-5
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DOI: https://doi.org/10.1007/s10336-011-0702-5