Extent and symmetry of tail moult in Amur Falcons

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.