Why are New Guinea's Cyclops Mountains poor in upland bird species?

Why do the Cyclops Mts., reputedly the fourth highest of New Guinea's ten outlying mountain ranges, support the second-lowest number of upland species? We surveyed Cyclops birds up to the summit. Despite observing most previously recorded upland species, we added only four to the prior list. Still unrecorded from the Cyclops are 22 upland species known from all or almost all other outliers. Most of those missing species are so common and/or conspicuous elsewhere that their apparent absence from the Cyclops is probably real. We compared upland species presence/absence in the Cyclops to elevational floors of upland species in the higher and much more species-rich Foja Mts. Unsurprisingly, Foja upland species with low Foja floors are better represented in the Cyclops than are species with Foja floors near or above the elevation of the Cyclops summit. But, surprisingly, the Cyclops are impoverished at low as well as at high elevations. The Cyclops summit avifauna consists of just eight species, most of them small forest understorey passerines endemic to New Guinea and absent from offshore islands, but abundant on the Cyclops summit. Only six subspecies are currently recognised as endemic to the Cyclops, none of them very distinctive—suggesting paradoxically that the Cyclops have not been difficult for upland species to reach, despite lacking so many apparently appropriate species. For most Cyclops upland species, three means of dispersal to the Cyclops readily suggest themselves: flying directly from other mountains, dispersal through the lowlands, or colonisation during Pleistocene cool phases with lowered climate zones. But those three mechanisms may not apply to 12 high-elevation Cyclops upland species endemic to New Guinea: we speculate about how they nevertheless arrived. We identify two factors contributing to Cyclops poverty in upland species: modest elevation (the summit is actually 250 m lower than the widely accepted value); and small area, hence frequent population extinctions resulting in low species number because of the species/area relationship.