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SUMMARYForaging strategies play a key role in breeding effort. Little is known, however, about their connection with hormonal and nutritional states, especially when breeding constraints vary. Here, we experimentally increased foraging costs and thus breeding constraints by handicapping Adélie penguins (Pygoscelis adeliae) with dummy devices representing 3-4% of the penguins' crosssectional area. We examined food-related stress (via plasma corticosterone concentration) and nutritional state (via metabolite levels). Concurrently, we investigated the use of ecological niches via the isotopic signature of red blood cells indicating the trophic position ( 15 N) and the spatial distribution ( 13 C) of penguins. Handicapped birds performed ~70% longer foraging trips and lost ~60% more body mass than controls and their partners. However, corticosterone levels and the nutritional state were unchanged. The isotopic signature revealed that males and females differed in their foraging behaviour: upper trophic levels contributed more in the males' diet, who foraged in more pelagic areas. Handicapped and partner birds adopted the same strategy at sea: a shift towards higher  13C values suggested that they foraged in more coastal areas than controls. This change in foraging decisions may optimize feeding time by decreasing travelling time. This may partly compensate for the presumed lower foraging efficiency of handicapped birds and for the energetic debt of their partners who had to fast ~70% longer on the nest. We propose that this flexible use of ecological niches may allow birds facing increased breeding constraints to avoid chronic stress and to minimize the impact on their body condition.