The birds of Norfolk & Lord Howe Islands and the Australasian South Polar quadrant :London :H.F. & G. Witherby,1928.biodiversitylibrary.org/page/58795369
This pair were engaging in some mutual preening, along with another pair just out of shot. This shot, taken early in the morning, gives a good impression of the deep chocolate coloured plumage of the species. Tern Island, Hawaii
""Dueling beaks?" --- this contact with the beaks continued for at least 15 minutes, interrupted only when one adult pointed its beak vertically at the sky."