Summary
Information concerning the strategy adopted by passerine migrants in crossing ecological barriers is still scant, especially as far as spring migration is concerned. With the aim of investigating the pattern of desert and sea crossing in spring, capture frequency, biometry and physical conditions of Garden Warblers ringed at 18 stations spread over the Western and Central Mediterranean were analysed as a function of the geographical position of the ringing sites. The patterns of increasing capture frequency and wing length with longitude, matching those recorded within the species' wide breeding range, suggest wide-front movements across the Sahara and Mediterranean. The progressively delayed trapping hour with latitude indicates that Garden Warblers migrate actively during the day. Despite the potentially favourable spring ecological conditions, the North African coast seems not to be used substantially to restore the reserves depleted during the desert crossing. According to recent estimates of flight range, fat levels recorded at our Mediterranean stations are compatible with the absence of refuelling in North Africa, assuming a fat load at departure of about 27 g or lower fat load and some tail wind assistance.
Zusammenfassung
Noch wie vor ist wenig bekannt, wie Singvögel große ökologische Barrieren überwinden. Dies gilt besonders für den Frühjahrszug. Um zu untersuchen, wie im Frühjahr die Wüste und das Mittelmeer überquert werden, haben wir Fangfrequenz, Biometrie und physische Kondition von Gartengrasmücken analysiert, die an 18 verschiedenen Fangstationen im zentralen und westlichen Mittelmeer gefangen wurden. Mit zunehmender geographischer Länge nahmen die Fanghäufigkeit und die Flügellänge zu. Diese Zunahmen entsprechen denen im Brutgebiet und zeigen, daß Gartengrasmücken die Sahara und das Mittelmeer in breiter Front überqueren. Die mit zunehmender geographischer Breite spätere Tagesstunde des Fangs zeigt, daß Gartengrasmücken auch tags ziehen. Ungeachtet der möglichen günstigen Rastbedingungen im Frühjahr in N-Afrika scheint die nordafrikanische Küste aber nicht zur Rast und Wiederauffüllung verbrauchter Energievorräte genutzt zu werden. Die gefundenen Fettwerte und die damit mögliche Schätzung der potentiellem Flugstrekke deuten eher darauf hin, daß die Gartengrasmücken südlich der Sahara mit einer Körpermasse von etwa 27 g oder weniger starten und unter Nutzung von Rückenwinden über Sahara und Mittelmeer ohne weiteres Auftanken ziehen.
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Grattarola, A., Spina, F. & Pilastro, A. Spring migration of the Garden Warbler(Sylvia borin) across the Mediterranean Sea. J Ornithol 140, 419–430 (1999). https://doi.org/10.1007/BF01650986
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DOI: https://doi.org/10.1007/BF01650986