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For many bird species, trade-offs in resource allocation become stark during incubation, when caring demands put into direct conflict their investment in reproduction versus survival. We demonstrate the critical importance of resource allocation, here measured indirectly as body mass, for incubation behavior in the Manx Shearwater (Puffinus puffinus), a biparentally-caring seabird. Using daily measurements of body mass from breeding pairs in combination with field observations and remotely collected behavioral data, we examined how changes in mass related to nest attendance and foraging behavior. We furthermore tested whether this differed between the sexes and between pairs of different breeding experience. We found that while body mass predicted the probability that incubating birds would choose to temporarily desert the nest, incubation shift duration was ultimately set by return of the foraging bird. The trip durations of foraging birds in turn were primarily dictated by their body mass reserves on departure from the nest. However, foragers appeared to account for the condition of the incubating partner by returning from sea earlier when their partner was in poor condition. This key finding suggests that decisions relating to resource allocation may be made cooperatively within the breeding pair during incubation. Our results contribute to understanding the mechanisms by which individuals regulate both their own and their partner's incubation behavior, with implications for interacting with fine-scale resource availability.

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