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We analysed 13C and 15N isotopic enrichment in Australasian harrier (Circus approximans) eggshell and two discarded harrier feathers from Motunau Island, a regionally important seabird breeding island. Among the prey remains found at the nest was a prion (Pachyptila sp.) wing fragment and a predated blue penguin (Eudyptula minor). We combined isotope data from the prey remains, plus potential prey items obtained from the mainland, to reconstruct harrier diet and evaluate incorporation of seabird nutrients. During egg material formation, blue penguins made up a major part of the female harrier’s diet. During autumn, when feathers were re-growing, the two feathers (which may or may not have been from different individuals) gave very different results. The feather with the more marine signature was growing when harrier diet included a significant proportion of blue penguin and/or fairy prion (Pachyptila turtur) material. Formation of the other feather may have occurred while harrier diet was primarily terrestrial. Our results are indicative of the usefulness of stable isotopic analysis in elucidating nutrient flows and contributions to animal diet.