Adult male Green Woodhoopoes (Phoeniculus purpureus) are only 5–8% larger than adult females in most linear measures but have 36% longer bills. Such sexual dimorphism may result from sexual selection, reproductive role division, or ecological separation. Here, we show that there is little evidence that sexual selection is currently acting on bill dimorphism in the Green Woodhoopoe. (1) Breeding males did not have longer bills than nonbreeding males. (2) There was no significant relationship between bill length and reproductive success of breeding males. (3) Although there was greater variation in male than in female bill length, the coefficient of variation (5.3%) fell within the range of those for naturally selected traits. (4) Although male bill length was found to be positively allometric with body mass, female bill length followed a similar relationship and there was no significant difference between the allometric slopes of the two sexes. Maintenance of the bill dimorphism by reproductive role division also seems unlikely when considering the nesting and provisioning characteristics of the species. We therefore conclude that the extreme sexual dimorphism in Green Woodhoopoe bill length is maintained by ecological separation to reduce foraging competition. We cannot, however, rule out the possibility that the sexual dimorphism initially evolved as a consequence of sexual selection.
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