Unexpected hydrogen isotope variation in oceanic pelagic seabirds

Hydrogen isotopes have significantly enhanced our understanding of the biogeography of migratory animals. The basis for this methodology lies in predictable, continental patterns of precipitation delta D values that are often reflected in an organism's tissues. delta D variation is not expected for oceanic pelagic organisms whose dietary hydrogen (water and organic hydrogen in prey) is transferred up the food web from an isotopically homogeneous water source. We report a 142 aEuro degrees range in the delta D values of flight feathers from the Hawaiian petrel (Pterodroma sandwichensis), an oceanic pelagic North Pacific species, and inquire about the source of that variation. We show delta D variation between and within four other oceanic pelagic species: Newell's shearwater (Puffinus auricularis newellii), Black-footed albatross (Phoebastria nigripes), Laysan albatross (Phoebastria immutabilis) and Buller's shearwater (Puffinus bulleri). The similarity between muscle delta D values of hatch-year Hawaiian petrels and their prey suggests that trophic fractionation does not influence delta D values of muscle. We hypothesize that isotopic discrimination is associated with water loss during salt excretion through salt glands. Salt load differs between seabirds that consume isosmotic squid and crustaceans and those that feed on hyposmotic teleost fish. In support of the salt gland hypothesis, we show an inverse relationship between delta D and percent teleost fish in diet for three seabird species. Our results demonstrate the utility of delta D in the study of oceanic consumers, while also contributing to a better understanding of delta D systematics, the basis for one of the most commonly utilized isotope tools in avian ecology.