Abstract
In altricial birds, parents are assumed to optimize the total food delivery to the brood given the time constraints set by self-feeding and food collecting. Older nestlings may require more food than younger ones, and nestlings may need more energy when their growth rate is higher. By video monitoring prey deliveries in ten nests of the Eurasian Kestrel (Falco tinnunculus), we examined whether parents adjusted feeding effort in relation to nestling age. Based on published data on the growth and energy intake of Kestrel nestlings, we predicted parental prey mass delivery to peak at a nestling age of 15–17 days. The prediction was supported. The decrease in provisioning rate during the later nestling stages was best explained by nestling age. However, we cannot be conclusive as to whether this was caused by a decrease in nestling food demand, or by a seasonal decrease in the availability of voles, the dominant prey. The change in provisioning was mostly an effect of a change in the number of prey items delivered. However, prey size also tended to decrease with increasing nestling age. This is opposite to what has been found in most non-raptorial altricial birds, and may have been caused by the ability of Kestrel parents to dismember large prey and thus overcome the gape size-restricted swallowing capacity of small nestlings, together with a need to provide smaller prey to older nestlings when they start to feed unassisted.
Zusammenfassung
Turmfalkeneltern ( Falco tinnunculus ) passen den Fütterungsaufwand an das Alter ihrer Nestlinge an.
Bei Nesthockern ist die Futterversorgung durch die Eltern entscheidend für Wachstum und Entwicklung der Jungvögel. Man nimmt an, dass die Eltern versuchen, ihrer Brut die in Anbetracht der zeitlichen Beschränkungen, denen sie durch die eigene Nahrungsbeschaffung und die Futtersuche unterliegen, größtmögliche Nahrungsmenge zu beschaffen. Ältere Nestlinge können mehr Nahrung benötigen als jüngere bzw. der Energiebedarf der Nestlinge kann in Phasen stärkeren Wachstums höher sein. Durch Videoaufzeichnung der übergebenen Beutestücke in zehn Turmfalkennestern wurde untersucht, ob die Eltern den Fütterungsaufwand in Abhängigkeit vom Nestlingsalter regulieren. Auf der Grundlage von publizierten Daten zu Wachstum und Energieaufnahme von Turmfalkennestlingen stellten wir die Vermutung auf, dass die Beutelieferungen durch die Eltern bei einem Nestlingsalter von 15–17 Tagen ein Maximum erreichen. Diese Annahme bestätigte sich. Die Abnahme der Versorgungsrate in den späteren Nestlingsstadien ließ sich besser durch das Nestlingsalter als durch die Jahreszeit erklären. Allerdings kann man nicht sicher sagen, ob dies durch eine Abnahme im Nahrungsbedarf der Nestlinge bedingt wurde, oder mit einem jahreszeitlichen Abfall in der Verfügbarkeit von Wühlmäusen, der Hauptbeute, zusammenhing. Die Änderung der Versorgungsmenge wurde hauptsächlich durch eine Änderung in der Anzahl der gelieferten Beutestücke bedingt. Allerdings nahm die Beutegröße mit steigendem Nestlingsalter tendenziell ab. Dies steht im Widerspruch zu den Befunden bei den meisten Nesthockern, die nicht zu den Beutegreifern gehören, und könnte auf die Fähigkeit letzterer zurückzuführen sein, große Beutetiere zu zerlegen und somit das Problem der durch die Rachengröße bedingten begrenzten Schluckkapazität kleiner Nestlinge zu umgehen, in Verbindung mit dem Bedarf an kleineren Beutetieren für ältere Nestlinge, wenn diese selbständig zu fressen beginnen.
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Acknowledgments
We thank Bjørn E. Foyn and Ole Petter Blestad for allowing us to film their Kestrel nest boxes, Geir A. Homme for help with the field work, Patricia L. Kennedy, Vidar Selås, Peter Sunde, and two anonymous referees for comments on the manuscript, Harry P. Andreassen for statistical advice, and Grethe Hillersøy for improving the English. The study was supported by the Directorate for Nature Management and the Hedmark County Governor.
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Communicated by T. Friedl.
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Steen, R., Sonerud, G.A. & Slagsvold, T. Parents adjust feeding effort in relation to nestling age in the Eurasian Kestrel (Falco tinnunculus). J Ornithol 153, 1087–1099 (2012). https://doi.org/10.1007/s10336-012-0838-y
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DOI: https://doi.org/10.1007/s10336-012-0838-y