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.

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... g 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 contrast to these seasonal changes, variations in the duration of sojourns into the colony, when penguins come independently to feed the chicks, can be attributed to the stages of the breeding cycle. Our setup also enables discriminating when the breed~ng failure is either due to poor food provisioning at sea or to the inability of the birds to minimize the depletion of their energy reserves when ashore. Thus, it is now possible to use breeding penguins as continuous indicators of changes in marine resources, on a time-scale of only days or weeks, while at the same time avoiding human disturbance by entering colonies and handling the birds.