From August to December, thousands of Black-necked Grebes Podiceps nigricollis concentrate during the flightless moult period in salt ponds in the Odiel Marshes, southern Spain, where they feed on the brine shrimp Artemia parthenogenetica. We predicted that because Black-necked Grebes moulted in a food-rich, predator-free environment, there would be no net loss of body mass caused by the use of fat stored to meet energy needs during remigial feather replacement (as is the case for some other diving waterbirds). However, because the food resource disappears in winter, we predicted that grebes moulting later in the season would put on more body mass prior to moult because of the increasing risk of an Artemia population crash before the moult period is completed. Body mass determinations of thousands of birds captured during 2000–2010 showed that grebes in active wing-moult showed greater mass with date of capture. Early-moulting grebes were significantly lighter at all stages than late-moulting birds. Grebes captured with new feathers post-moult were significantly lighter than those in moult. This is the first study to support the hypothesis that individual waterbirds adopt different strategies in body mass accumulation according to timing of moult: early-season grebes were able to acquire an excess of energy over expenditure and accumulate fat stores while moulting. Delayed moulters acquired greater fat stores in advance of moult to contribute to energy expenditure for feather replacement and retained extra stores later, most likely as a bet hedge against the increasing probability of failing food supply and higher thermoregulatory demands late in the season. An alternative hypothesis, that mass change is affected by a trophically transmitted cestode using brine shrimps as an intermediate host and Black-necked Grebes as final host, was not supported by the data.