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The “cost-benefit” hypothesis states that avian body organs show mass changes consistent with the trade-off between their functional importance and maintenance cost, which may vary throughout the annual cycle. Flightless moulting common scoter Melanitta nigra in Danish marine waters select rich undisturbed offshore feeding areas lacking predators, suggesting active feeding during moult. We tested four predictions relating to organ size during flightlessness in moulting male common scoter under this hypothesis. Namely that (i) pectoral muscles would show atrophy followed by hypertrophy, but that there would be no change in (ii) leg muscles and heart (the locomotory architecture required to sustain diving for food), (iii) digestive organs and liver (required to process food), or (iv) fat deposits (because birds could fulfil daily energy requirements from locally abundant food resources). Dissection of scoters collected at different stages during wing moult south of the Danish island of Læsø provided data on organ size that were consistent with these predictions. Pectoral muscle mass showed a c.23% atrophy during the middle of the flightless period relative to that at the end of moult. There was no significant loss in leg muscle, heart, digestive organs (except gizzard mass), liver, fat reserves or body mass with remigial growth. These findings are consistent with the hypothesis that common scoter moult in a rich feeding area, and rely on their diet to meet the nutritional requirements of remigial moult. These results differ in detail from those of a similar study of terrestrial feeding moulting greylag geese Anser anser, but because of the widely differing ecology of the species concerned, both sets of findings provide strong support for the hypothesis that variations in phenotypic plasticity in size of fat stores, locomotor and digestive organs can be interpreted as evolutionary adaptations to meet the conflicting needs (feather growth, nutritional challenges and predator avoidance) of the flightless moult period in different Anatidae species.