Stomach oil, a complex mixture of neutral dietary lipids, is a unique attribute of seabirds in the order Procellariiformes. With the exception of diving-petrels, all procellariiforms produce stomach oil and feed it to their young. We conducted an interspecies cross-fostering experiment on Bird Island, South Georgia, that was designed to reveal how the presence or absence of stomach oil in meals fed to young seabirds influences their growth, development, and survival. Hatchling South Georgia Diving-Petrels (Pelecanoides georgicus), a species that lacks stomach oil, were switched with hatchling Antarctic Prions (Pachyptila desolata), a species that feeds its young stomach oil. Diving-petrel foster parents did not successfully raise prion nestlings, presumably due to the absence of stomach oil in meals fed to nestlings. Prion foster parents successfully raised diving-petrel nestlings to fledging, but growth rates were lower, nestling fat reserves were lower, and fledging was delayed compared with controls. These results suggest that stomach oil is an essential dietary component for prion nestlings to meet their energy requirements, but diving-petrel nestlings apparently cannot efficiently assimilate stomach oil. This experiment supports the hypothesis that the production of stomach oil is an adaptation that allows breeding seabirds to enhance provisioning rates of energy to the nest, while foraging on a distant and dispersed food supply.
