Differences in biometrics and moult of non-breeding Red Knots Calidris canutus in southern Africa and Scotland reflect contrasting climatic conditions

摘要

We describe the migration, biometrics and moult of Red Knot Calidris canutus canutus in southern Africa and compare them with the biometrics and moult of Calidris canutus islandica in northern Europe to examine possible adaptations to different environments during the non-breeding season. Northward and southward migration of C. c. canutus took place along the coast of Western Europe and there was one recovery in West Africa (Mauritania), suggesting a coastal migration round West Africa rather than migration across the Sahara, as recorded in other waders. Adult Knots in South Africa had no additional fattening in November-January (fat index of 7%), in contrast to C. c. islandica wintering in Britain. This is consistent with the theory that extra fat is required only where food shortages are likely. The bills of canutus were longer than those of islandica but their wings were shorter, confirming the sub-specific assignments and origin of this population. The average duration of primary moult in South Africa was 95 days, shorter than that of other Arctic-breeding waders that moult in South Africa, but longer than of islandica moulting in Scotland (77 days). Mean starting and completion dates were 20 July and 5 October for islandica and 25 October and 28 January for canutus. The timing and duration of primary moult for these two subspecies suggest that waders need to complete moult before the northern winter when food supplies are limited, whilst waders in benign climates face no such pressures. First-year canutus either retained old primaries for much of their first year or had a partial moult of inner or other primaries. Adults departed on northward migration in mid-April, having attained a mean departure mass of c. 190 g (maximum 232 g). The mean fat index at this time was 24% (maximum 29%) and the fat-free flight muscle mass increased. The predicted flight range of 4000 km falls short of the distance to the first likely refuelling site in West Africa, suggesting that birds rely on assistance from favourable winds.