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Trends in sea-bird population sizes reflect changes in marine resources, but are only vislble after years. Sea-blrds fast when breeding ashore and, therefore, breeding success largely depends upon body fuels accumulated at sea and food stored in the stomach for chicks. Using an automatic setup for identification and weighing of breeding king penguins Aptenodytespatagonicus, we demonstrate seasonal differences in the daily gain in body mass and duration of foraging trips of breeders at sea. Taking into account already available information, our data indicate that it takes longer for the breeders to obtain food when marine resources are decreas~ ng. The overall gain in body mass of the birds at sea is unchanged. However, they accumulate larger body fuel reserves, which therefore increases their energetic safety margin at predictable times of lower food availability but reduces food brought back to the chicks. In …