Piersma T. (1988) Breast muscle atrophy and constraints on foraging during the flightless period of wing moulting Great Crested Grebes. ARDEA 76 (1): 96-106

Simultaneously wing moulting birds face a large metabolic demand for feather synthesis. To explain the low body and breast muscle masses of Anseriform birds during simultaneous wing moult, two causal explanations, the 'nutritional stress' and the 'use-disuse' hypotheses, have been proposed. This paper examines both hypotheses on the basis of data on the changes in body composition, food intake and feeding activity of a non-Anseriform bird during its simultaneous wing moult: Great Crested Grebes on Lake IJsselmeer, The Netherlands. In the wing moulting period, Great Crested Grebes have the lowest body mass of the entire non-breeding season. This is due to a small fat store and a low mass of the breast muscles. In the course of wing moult, fat mass does not change .much, but breast muscle mass decreases by 15% just before and/or during the very start of wing moult, with a further 7% reduction in the remaining first half. Leg muscle mass remains much the same (or possibly becomes somewhat smaller). Although the composition of the diet (which mainly consists of Smelt) does not change during moult, the stomach contents indicate that halfway through wing moult Great Crested Grebes have a very low daily food intake. Field observations suggest that this category of grebes spends less time on the feeding areas, and dives less often or less deep. The close correlations between mass and inferred activity of leg and breast muscles of Great Crested Grebes, support the use-disuse hypothesis. The data do not exclude the possibility that the mobilized breast muscle proteins are used for feather synthesis. For a better understanding, studies on the levels of exercise allowing muscles to increase or decrease in size, either or not under additional hormonal or nervous control are required. The fact that Great Crested Grebes eat little halfway through wing moult is hypothesized to be due to the grebes' trying to minimize the risk of breaking the exposed and fragile growing flight feathers underwater.