We investigated the adaptations of primary moult to conditions on the nonbreeding areas in the Curlew Sandpiper Calidris ferruginea, an Arctic breeding, long-distance migratory wader (shorebird). Parameters of moult (start date, duration, and standard deviation) and how they are affected by rainfall patterns were estimated using and extension of the Underhill—Zucchini (1988) moult models which allows the inclusion of covariates. The estimated start date and duration of moult in Kenya was 2 October and 128 days, respectively; start date was negatively correlated with the rainfall in June. In India, the estimated start date of moult was 15 August, duration 107 days and start of moult was positively correlated to the rainfall in August. Circumstantial evidence also suggests that the rate of growth of the primaries was affected by food supply at the nonbreeding areas. The timing of moult to coincide with rainfall periods is, perhaps, an adaptation to carry out this energy demanding activity during periods of food abundance. This study shows that in these populations of the Curlew Sandpiper, the timing of moult is a direct response to environmental variation and might be used to measuring the impacts of climate change on life-history traits of migratory birds.