Resource allocation underlies parental decision-making during incubation in the Manx shearwater

Examining resource allocation is fundamental to understanding the relationships between a species’ behaviour and its life history. Furthermore, for biparentally-caring animals, examining the relative investment decisions made by members of a breeding pair can give insight into the extent and nature of cooperative care. As a key measure of resource availability, examining body mass changes can help elucidate the ways in which parents balance their allocation. In birds, these trade-offs become particularly stark during incubation, as maintaining constant egg warming usually requires one parent to fast. This period therefore represents a key opportunity to investigate investment decisions. We took daily measurements of body mass from breeding Manx shearwaters, a biparentally-caring seabird, during incubation, and related this to measures of nest attendance and behaviour collected using field observations and miniaturised biologgers. We investigated how changes in body mass related to the decisions made at the nest and at sea, whether this differed between the sexes, and whether pair experience influenced incubation behaviour. 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, returning from sea earlier when their partner was in poor condition. Our results contribute to understanding the mechanisms by which individuals regulate both their own and their partner's incubation behaviour, with implications for interacting with fine-scale resource availability.