Trade‐offs between moult and fuelling in migrant birds vary with migration distance and the environmental conditions they encounter. We compared wing moult and fuelling at the northern and southern ends of migration in two populations of adult Common Whitethroats Sylvia communis. The western population moults most remiges at the breeding grounds in Europe (e.g. Poland) and migrates 4000–5000 km to western Africa (e.g. Nigeria). The eastern population moults all remiges at the non‐breeding grounds and migrates 7000–10 000 km from western Asia (e.g. southwestern Siberia) to eastern and southern Africa. We tested the hypotheses that: (1) Whitethroats moult their wing feathers slowly in South Africa, where they face fewer time constraints than in Poland, and (2) fuelling is slower when it coincides with moulting (Poland, South Africa) than when it occurs alone (Siberia, Nigeria). We estimated moult timing of primaries, secondaries and tertials from moult records of Polish and South African Whitethroats ringed in 1987–2017 and determined fuelling patterns from the body mass of Whitethroats ringed in all four regions. The western population moulted wing feathers in Poland over 55 days (2 July–26 August) at a varying rate, up to 13 feathers simultaneously, but fuelled slowly until departure in August–mid‐September. In Nigeria, during the drier period of mid‐February–March they fuelled slowly, but the fuelling rate increased three‐fold in April–May after the rains before mid‐April–May departure. The eastern population did not moult in Siberia but fuelled three times faster before mid‐July–early August departure than did the western birds moulting in Poland. In South Africa, the Whitethroats moulted over 57 days (2 January–28 February) at a constant rate of up to nine feathers simultaneously and fuelled slowly from mid‐December until mid‐April–May departure. These results suggest the two populations use contrasting strategies to capitalize on food supplies before departure from breeding and non‐breeding grounds.
Appendix S1. Fig. S1. Moult timing and sequence of each wing feather for western (Poland–Nigeria) and eastern (Siberia–South Africa) Whitethroats.
Fig. S2. The number of wing flight feathers growing simultaneously with the feather on the X‐axis for Common Whitethroats in Poland and in South Africa.
Table S1. Mean relative mass of flight feathers in adult Common Whitethroats expressed as a percentage of the total mass of all wing feathers treated as 100%, and as percentage of the total mass of all primaries (P1–P9) treated as 100%.
Table S2. Moult sequence and moult parameters of separate wing feathers for adult Common Whitethroats caught in July–October 2013–2016 in Poland.
Table S3. Moult sequence and moult parameters of separate wing feathers for adult Common Whitethroats caught in November–April 1987–2017 in South Africa.
Table S4. Underhill–Zucchini moult models used to determine the effect of region where moult takes place (see Fig. 1) on moult parameters estimated for all primaries, secondaries and tertials jointly in adult Common Whitethroats caught in July–October 2013–2016 in Poland and in November–April 1987–2017 in South Africa.
Table S5. Mean wing lengths of Whitethroats caught in the four study regions (Fig. 1), considering the moult status of measured wings.
Table S6. Comparison of primary moult rates estimated by Underhill–Zucchini models for Whitethroats in Poland and in South Africa (Tables S2 and S3) with those for other insectivorous passerine migrants.