WASSINK G (2009) Ageing and identifying individual Eagle Owls Bubo bubo on the basis of flight feathers and moult patterns. LIMOSA 82 (2): 59-67.

Timing and sequence of flight feather moult of a captive female Eagle Owl called 'Svenja' were studied. Feathers were only replaced between May and September. In the first summer (one year after hatching) only a few inner secondaries were renewed (Fig. 1). In the second summer also the first primaries were replaced. It took until the fourth summer before all juvenile flight feathers had been moulted. Individual feathers were generally replaced every second to third year. The general moult pattern was similar to a captive Eagle Owl from Germany (Glutz von Blotzheim & Bauer 1980), although the rate of moult was much lower in the current study. This difference is likely due to the fact that this owl was trained on a daily basis (flying for at least 30 minutes per day), and hence her speed of moult more closely resembles the natural situation. The length of the feathers increased with every generation (Table 1, Fig. 2). Feathers also slightly changed in appearance, allowing to determine the age of an owl from a moulting feather. For example, feathers from later generations had fewer but broader and darker bands. The pattern of bands and stripes was rather consistent between feather generations. Feathers can thus also be used to identify individual birds. A few examples are presented to illustrate how moult patterns can be used to age Eagle Owls.

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