Abstract
Declining body size has been suggested to be a response of animals to global warming, but analyses of time series have led to contradictory results. One problem is that each trait related to body size may vary in response to factors other than temperature and independently of size. We analyse trends of three morphological traits of a passerine bird species: the Stonechat Saxicola torquata. Wing lengths were increasing and tail length mostly decreasing between 1989 and 2012. Variation in tarsus length showed no consistent trend. Wing length increased with increasing temperature. Concomitant decreasing tail length suggests, however, that increasing wing length cannot be explained by increasing temperatures during the study period. As tarsus length is a surrogate for overall size, we argue that there was no detectable trend in body size. Wing and tail length are related to flight performance, and increasing wing and decreasing tail length could be indicative of selection for more effective flight, related to either longer migration distances or increased predation pressure. The first scenario is unlikely given the strong suggestions of reduced migratory activity in birds as a response to climate change. The density of the Sparrowhawk Accipiter nisus recently increased almost tenfold in the study area, but the hypothesis of changing morphology as a response to increasing predation pressure remains to be tested. Our study suggests, however, that linking fluctuating lengths in single morphological traits to body size change as a response to global warming may be premature when alternative hypotheses are not considered.
Zusammenfassung
Morphologische Veränderungen beim Schwarzkehlchen Saxicola torquata : eine Folge des Klimawandels?
Die Abnahme der Körpergröße wurde als eine der möglichen Reaktion von Tieren auf die globale Erwärmung angesehen, entsprechende Studien kamen jedoch zu widersprüchlichen Ergebnissen. Ein Problem dabei ist, dass verschiedene angenommene Größenindikatoren in Abhängigkeit von anderen Umweltfaktoren und unabhängig von Temperaturen fluktuieren können. Wir untersuchten die Trends von drei morphologischen Variablen beim Schwarzkehlchen Saxicola torquata. Zwischen 1989 und 2012 nahm die Flügellänge zu und die Länge der Steuerfedern ab, während die Tarsuslänge ohne klaren Trend fluktuierte. Die Flügellänge nahm mit steigenden Temperaturen zu. Die mit steigenden Temperaturen kürzer werdenden Steuerfedern zeigen jedoch, dass die zunehmende Flügellänge nicht durch steigende Temperaturen erklärt werden kann. Da die Tarsuslänge als Größenmaß gilt, ist davon auszugehen, dass kein Veränderung der Körpergröße statt gefunden hat. Längere Flügel und kürzere Steuerfedern deuten die Selektion auf einen effektiveren Flug an, als mögliche Anpassung an einen längeren Zugweg oder steigenden Prädationsdruck. Ersteres erscheint unwahrscheinlich im Hinblick auf die generell abnehmenden Zugdistanzen von Vögeln als Reaktion auf den Klimawandel. Die Dichte des Sperbers Accipiter nisus hat im Untersuchungsgebiet annähernd zehnfach zugenommen. Ob morphologische Änderungen als Anpassung an einen gestiegenen Prädationsdruck stattfinden, muss aber noch geprüft werden. Die Studie weist jedoch darauf hin, dass es kritisch ist, Veränderungen einzelner morphologischer Variablen als Anpassungen an den Klimawandel zu interpretieren, wenn alternative Hypothesen nicht berücksichtigt werden.
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Acknowledgments
The Institute of Avian Research “Vogelwarte Helgoland” provided H.F. with Helgoland rings. R. Schlenker and K.-H. Siebenrock, Max-Planck-Institute for Ornithology, Radolfzell, provided H.F. with some trapping equipment. The farmers of the Düffel and the Heubach-Lowland generously allowed working on their estates. R. Fischer, Bocholt-Loeverick, kindly provided the data from his private weather station. The data analysis for this project was supported by a grant from the Deutsche Ornithologen-Gesellschaft (DO-G).
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Salewski, V., Hochachka, W.M. & Flinks, H. Changes in Stonechat Saxicola torquata morphology: a response to climate change?. J Ornithol 155, 601–609 (2014). https://doi.org/10.1007/s10336-014-1042-z
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DOI: https://doi.org/10.1007/s10336-014-1042-z