Notes on appearance and speculated behavior of the O'ahu 'O'o (meliphagidae)

The O'ahu 'O'o (Moho apicalis) was last collected in 1837, and is one of the rarest Hawaiian birds in museum collections. Despite the cultural importance of the 'O'o as a source of yellow feathers for Hawaiian featherwork, next to nothing is known about this species. A pair of these extinct honeyeaters at the Museum fuer Naturkunde der Humboldt-Universitaet in Berlin have conspicuous yellow bare orbital rings, features not previously noted or illustrated. Individuals apparently varied in the expression of this character, because six other specimens did not have the yellow ring. Possible sources of variation include age, sex, agonistic state, and breeding condition. That this character varies with age is suggested by its presence in a reduced form in juveniles of other 'O'o species. Increased sexual dimorphism of the modified tails relative to body size in this and other 'O'o species indicates sexual selection on tails either from intrasexual aggression or intersexual mate choice. Patterns of contrasting yellow feather tufts differ between birds from different islands, but not between sexes, suggesting that males and females experienced similar evolutionary pressures for this feature. The 'O'o's tail may have evolved under the influence of sexual selection on males, and the conspicuous yellow feather tufts by social selection experienced equally by both sexes.