Flexibility in food resource allocation in parents and selectivity for offspring: variations in δ15N and δ13C values during breeding of the blue-footed booby

It is common for marine animals to be selective in the amount and quality of foods during reproduction due to the higher energetic demands. As central place foragers, seabirds are often selective with the prey species they deliver to their offspring. We evaluated trophic segregation between parents and their offspring in blue-footed booby (Sula nebouxii) by measuring stable isotope ratios (δ15N and δ13C) in blood samples during two breeding seasons and at different reproductive stages (incubation, early and late chick rearing). Additionally, we examined if δ15N values from chicks were correlated with their condition as reflected in blood alkaline phosphatase (ALP) levels. δ15N values increased and δ13C values decreased in adults as the breeding season progressed, indicating differences in foraging regions or the assimilated prey. δ15N values differed significantly between sexes; males had higher δ15N values than females during all reproductive stages sampled during the 2011 season, whereas in 2012 the difference between sexes was only observed during incubation. Offspring had higher δ15N values and lower δ13C values than adults, suggesting that, when feeding their chicks, parents feed them with prey from higher trophic levels and possibly from more oceanic environments. ALP levels, a proxy for bone growth and condition, measured in the offspring were positively correlated with δ15N values during the chick-rearing period. Although the diet of parents differed between reproductive stages, a multi-source Bayesian mixing model indicated that their primary prey were Pacific anchovy and halfbeaks, with a combined modal fractional contribution of 50–70% of the diet. The diet of the offspring was dominated by Pacific anchovy (≈36%), which was also the prey with the highest lipid content (C:N ratio 5.57), whereas the contributions of other fish species (or groups) were similar (17–24% each). Our findings suggest that parents actively feed their young with high-quality prey, which seems to increase some of the individual fitness components associated with better growth.