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Sexual differences in vocalisations of the Vanuatu petrel (Pterodroma occulta) are described. Qualitative differences in burrow calls could be used to sex adults with 63-100% accuracy in listening experiments. Males sounded “clear” and females sounded “hoarse”. Higher accuracy is possible with the aid of spectrograms. Playback experiments demonstrated a male-bias in responses of incubating Vanuatu petrels to “war-whooping” and flight calls. Acoustic methods have practical and ethical advantages over handling breeding petrels and further studies of the vocal behaviour of gadfly-petrels are encouraged.