Abstract
Farmland bird populations in Europe are shrinking, largely due to modern agriculture practices. In grasslands, the shift to early mowing is believed to be particularly responsible for this decline because it is interwoven with breeding time—a change that birds in general have not adapted to. We studied the post-fledging survival of the Whinchat, an altricial grassland passerine. Based on a sample of 74 radiotagged young Whinchats, we confirmed that they fledge at 13–15 days. Twenty-four fledged birds died, while 18 of them were depredated. The survival probability of young Whinchats, calculated using Kaplan–Meier estimates, was lowest during the first day after fledging, and slowly leveled off later. In total, the probability that a juvenile survived the first month after fledging was 0.52. During the first days after fledging, all chicks were found hiding on the ground among the grass stubble. The generalized linear mixed model predicts that half of the fledglings at 20 days old and 20% at 22 days old (8–10 days after fledging) will not escape but will instead remain still when approached by a threat. Sixteen depredations occurred during this period, all by mammal predators. Afterwards, the fledglings changed their strategy, escaping through flight. After 12 days, more and more of the chicks were found exposed on top of the grass or on the bush instead of hiding on the ground. The transition from sit and hide to escape behavior reduced the predation rate considerably. Only two chicks were depredated later, both by an avian predator. Due to the initial predator avoidance strategy, the current conservation measure (to postpone mowing until the chicks in 80% of all nests have fledged) is not sufficient to halt population decline on improved meadows. We suggest that at least 10 days more, and possibly even 14 days more, are necessary to maintain viable Whinchat populations.
Zusammenfassung
Die Praktiken in der modernen Landwirtschaft führen in Europa zu einer Abnahme der Vogelpopulationen auf Ackerland. Vor allem der Wechsel hin zu früher Gras-Mahd wird dafür verantwortlich gemacht, weil diese immer häufiger in die Brutzeit fällt—eine Veränderung, an die sich die Vögel nicht anpassen konnten. Wir untersuchten das Überleben in der Zeit direkt nach dem Flüggewerden bei Braunkehlchen, einem Nesthocker unter den auf Wiesen lebenden Singvögeln. Anhand von 74 mit Sendern versehenen, jungen Braunkehlchen konnten wir bestätigen, dass sie im Alter von 13 bis 15 Tagen flügge werden. 24 der markierten Vögel fielen aus, weil sie starben; 18 von ihnen wurden erbeutet. Die Überlebenswahrscheinlichkeit der jungen Braunkehlchen wurde mit der Kaplan-Meier-Methode eingeschätzt; sie war in den ersten fünf Tagen nach dem Flüggewerden am niedrigsten und pendelte sich danach langsam ein. Insgesamt lag die Überlebenswahrscheinlichkeit für den ersten Monat nach Flüggewerden bei 0,52. In den ersten Tagen versteckten sich alle Küken zwischen den Grasstoppeln am Boden. Das generalisierte lineare gemischte Modell (GLMM) sagt voraus, dass 50% der Küken am 20. Tag nach Schlüpfen, 20% im Alter von 22 Tagen (8–10 Tage nach Flüggewerden), bei einer Bedrohung nicht fliehen, sondern sich ganz ruhig verhalten. Sechzehn der Tiere wurden in dieser Zeit aufgestört, allesamt von Säugetieren, und änderten daraufhin ihre Strategie, indem sie aufflogen und flohen. Nach 12 Tagen gab es immer mehr Küken, die auf den Grasbüscheln zu sehen waren, anstatt sich am Boden zu verstecken. Dieser Wechsel vom sich Hinhocken und Verstecken zu einem Fluchtverhalten senkte die Rate der erbeuteten Küken beträchtlich. Nur zwei Küken wurden anschließend noch erbeutet, beide von Raubvögeln. Wegen dieser frühen Räubervermeidungs-Strategie reicht die derzeitige Schutzmaßnahme, das Mähen der Wiesen aufzuschieben, bis die Küken von 80% aller Nester flügge sind, nicht aus, um den Rückgang der Populationen auf den genutzten Wiesen aufzuhalten. Wir schlagen vor, dass mindestens 10, besser aber noch 14, zusätzliche Tage gewartet werden sollte, um lebensfähige Braunkehlchen-Populationen zu erhalten.
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Acknowledgments
We are very grateful to Dr. Urška Koce for her help during the field work, Dr. R.H. Pain (UK) for his comments and linguistic suggestions on the draft, as well as Dr. M. Grüebler, Dr. F. Korner and an anonymous reviewer for their valuable comments. The study was conducted in accordance with current Slovenian laws and was supported by the Slovenian Research Agency.
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Communicated by P. H. Becker.
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Tome, D., Denac, D. Survival and development of predator avoidance in the post-fledging period of the Whinchat (Saxicola rubetra): consequences for conservation measures. J Ornithol 153, 131–138 (2012). https://doi.org/10.1007/s10336-011-0713-2
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DOI: https://doi.org/10.1007/s10336-011-0713-2