VARIABLE SEXUAL ORNAMENTS IN SCARLET-TUFTED MALACHITE SUNBIRDS (NECTARINIA-JOHNSTONI) ON MOUNT KENYA

order to be elaborated by sexual selection, sexual ornaments must vary perceptibly and genetically among individuals in natural populations. Rather little is known about ornament Variation in monogamous species, in which sexual selection should act more weakly than in polygynous species. We report phenotypic variation in feather ornament size (elongated tails and pectoral tufts) and body size in the scarlet-tufted malachite sunbird Nectarinia johnstoni, a monogamous, sexually dimorphic nectarivore of East African alpine zones. Full-expressed male ornaments are highly significantly more variable (CVs = 12-29%) than are skeletal and wing measures primarily affected by natural selection (CVs = 2-4%). Female sunbirds have pectoral tufts which are significantly (22-25%) smaller than those of adult males, but more variable (CV female = 21-22%, CV male = 12-15%), and more variable than body size. Among males with fully-grown ornaments, those with longer tails tend to have longer wings and wider tufts. The high variation in fully-grown ornaments in malachite sunbirds is consistent with the view that the ornaments are condition-dependent sexual signals. Finally, we review studies of feather ornament variation to date, and show that ornaments are much more variable in monogamous than non-monogamous species, apparently due to the relatively weak pressure of sexual selection.