Abstract
Amur Falcons Falco amurensis undergo one of the most extreme migrations of any raptor, crossing the Indian Ocean between their Asian breeding grounds and non-breeding areas in southern Africa. Adults are thought to replace all their flight feathers on the wintering grounds, but juveniles only replace some tail feathers before migrating. We compare the extent and symmetry of flight feather moult in a large sample of Amur Falcons killed at communal roosts during two hailstorms in KwaZulu-Natal, South Africa in March 2019, shortly before their northward migration. Most adults had completed replacing their remiges, with only a few still growing 1–3 feathers (mainly secondaries), but most were still growing their tail feathers. Juveniles only replaced tail feathers. Moult typically was distal from the central rectrices, but 25% of adults and 1% of juveniles replaced the outer tail first, and a few individuals exhibited other moult patterns (simultaneous moult across the tail, or among the inner and outer feathers). These different moult strategies were independent of sex. Adults that replaced the outer tail first typically had replaced a greater proportion of the rectrices (mean ± SD; 0.81 ± 0.19) than adults starting from the central tail (0.17 ± 0.08). Proportionally fewer distal moulting adults were killed on 9 March than 21 March, resulting in the average proportion of rectrices replaced by adults decreasing between the two storm events from 0.52 ± 0.26 to 0.43 ± 0.23. By comparison, juvenile tail moult increased from 9 March (0.34 ± 0.18) to 21 March (0.40 ± 0.15). Overall, the probability of replacement for T1 was similar for adults (0.82) and juveniles (0.83), but adults were more likely to have replaced T2–6 (0.40–0.45) than juveniles (0.18 for T2 and 0.04–0.07 for T3–6). Asymmetry in tail moult was greater at T1 for adults (15%) than juveniles (10%), but asymmetry for T2 to T6 was greater in juveniles (3–10%) than adults (1–4%), especially given the greater probability of feather replacement in adults. Despite these differences, the degree of asymmetry was less than expected by random replacement across all rectrices in both age classes. Interestingly, moult tended to be more advanced on the left than right side of the tail. The extent of tail moult was correlated with body condition in adults and juveniles, suggesting that moult pattern might be used as an indicator of fitness in falcons.
Zusammenfassung
Ausmaß und Symmetrie der Schwanzmauser bei Amurfalken Unter allen Greifvögeln absolvieren die Amurfalken (Falco amurensis) eine der extremsten Wanderungen, auf der sie den Indischen Ozean zwischen ihren asiatischen Brutgebieten und den Nicht-Brutgebieten im südlichen Afrika überqueren. Die adulten Vögel ersetzen vermutlich im Winterquartier alle ihre Federn, wohingegen die Jungtiere vor dem Zug nur einige Schwanzfedern ersetzen. Wir vergleichen das Ausmaß und die Symmetrie der Flugfedermauser bei einer großen Stichprobe von Amurfalken, die an ihren gemeinsamen Schlafplätzen während zweier Hagelstürme in KwaZulu-Natal, Südafrika, im März 2019 kurz vor ihrem Zug nach Norden getötet wurden. Die meisten der adulten Tiere hatten den Austausch ihrer Flügelfedern abgeschlossen, nur bei einigen wenigen wuchsen noch 1–3 Federn (vor allem Handschwingen), bei den meisten aber noch die Schwanzfedern. Die Jungtiere ersetzten nur die Schwanzfedern. Die Mauser verlief typischerweise distal von den zentralen äußeren Schwanzfedern, aber 25% der adulten Tiere und 1% der Jungtiere ersetzten zuerst die äußeren Schwanzfedern, und ein paar Einzeltiere zeigten andere Mausermuster (eine gleichzeitige Mauser entweder quer über den Schwanz oder zwischen den inneren und äußeren Federn). Die unterschiedlichen Strategien in der Mauser waren unabhängig vom Geschlecht. Erwachsene Tiere, die zuerst die äußeren Schwanzfedern ersetzten, hatten in der Regel bereits den größeren Teil der Schwanzfedern ersetzt (Mittelwert ± SD; 0,81 ± 0,19), im Gegensatz zu den Adulten, deren Mauser in der Mitte des Schwanzes begann (0,17 ± 0,08). Am 9. März wurden verhältnismäßig weniger mausernde Adulte getötet als am 21. März, was dazu führte, dass bei den Adulten der durchschnittliche Anteil der ersetzten äußeren Schwanzfedern zwischen den beiden Hagelstürmen von 0,52 ± 0,26 auf 0,43 ± 0,23 abnahm. Im Vergleich dazu nahm die Schwanzmauser bei den Jungtieren vom 9. März (0,34 ± 0,18) bis zum 21. März (0,40 ± 0,15) zu. Insgesamt war die Wahrscheinlichkeit des Austauschs der T1 (T1 = Schwanzfeder 1) bei Adulten (0,82) und Jungtieren (0,83) ähnlich, aber die Adulten zeigten eine höhere Wahrscheinlichkeit, die T2-6 auszutauschen (0,40–0,45) als Jungtiere (0,18 für T2 und 0,04–0,07 für T3-6). Die Asymmetrie der Schwanzmauser war für T1 bei den Adulten größer (15%) als bei den Jungtieren (10%), aber die Asymmetrie bei T2 bis T6 war bei Jungtieren (3–10%) größer als bei Erwachsenen (1–4%), insbesondere angesichts der größeren Wahrscheinlichkeit einer kompletten Mauser bei den Adulten. Trotz dieser Unterschiede war der Grad der Asymmetrie geringer als bei einer zufälligen Mauser über alle äußeren Schwanzfedern in beiden Altersklassen zu erwarten wäre. Interessanterweise war die Mauser auf der linken Seite des Schwanzes tendenziell weiter fortgeschritten als auf der rechten. Das Ausmaß der Schwanzmauser korrelierte mit der Körperverfassung der Adulten und Jungvögeln, was darauf hindeutet, dass das Muster des Mauserverlaufs als Indikator für die Fitness von Falken verwendet werden könnte.
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Acknowledgements
We thank management, staff, interns and volunteers of Durban Natural Science Museum, Durban, South Africa for help and logistics. The staff of FreeMe KwaZulu-Natal Wildlife Rehabilitation Centre, especially Wade Whitehead and Tammy Caine, as well as Ben Hoffman of Raptor Research and Meyrick Bowker are thanked for assistance in making the falcon carcasses available. OEA’s study was funded by the South African National Research Foundation (Grant no. 110950), the DST-NRF Centre of Excellence at the FitzPatrick Institute of African Ornithology and a UCT International/Refugee Scholarship for Postgraduate Students. We also thank Peter Pyle, Sievert Rohwer and an anonymous reviewer for helpful suggestions on the manuscript.
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OEA: manuscript writing, statistical analysis, and data collection; DGA: manuscript revisions and data collection; BZ: manuscript revisions and data collection; WD: data collection; PGR: manuscript writing and revisions, data collection and analysis, supervision. OEA, DGA, BZ, and PGR discussed the results and commented on the manuscript.
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Adekola, O.E., Allan, D.G., Bernitz, Z. et al. Extent and symmetry of tail moult in Amur Falcons. J Ornithol 162, 655–667 (2021). https://doi.org/10.1007/s10336-021-01874-0
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DOI: https://doi.org/10.1007/s10336-021-01874-0