Avifauna of the Adelbert Mountains, New Guinea: why is Fire-maned Bowerbird Sericulus bakeri the mountains' only endemic bird species?

Abstract

The Adelbert Mountains, one of ten outlying ranges along New Guinea's north and north-west coasts, surprised ornithologists when their first exploration by Western scientists yielded the striking endemic Fire-maned Bowerbird Sericulus bakeri. It was then another surprise when further exploration revealed no other distinctive endemic. We summarise previous Adelbert studies and our four explorations including a survey of the highest summit. A total of 71 upland species has been recorded from the Adelberts, all of them also present as the same species or (in the case of S. bakeri) same superspecies on other outliers. The Adelberts are exceptional among low-elevation outliers in harbouring populations of seven upland species shared only with much higher outliers. The Adelberts are unique in supporting populations of ten upland species compressed at the highest elevations into a narrow elevational band below the summit. The elevational floors of those species lie a much shorter distance below the summit than for any species on any other outlier. In explanation, we propose the hypothesis that, among outliers, the Adelberts are especially accessible to colonisation by upland species from other upland areas, with two consequences: endemism is almost non-existent in the Adelberts except S. bakeri; and high-elevation populations of the Adelberts may be subsidised by colonists from other upland areas. The highest-elevation populations may have disappeared during the mid-Holocene hypsithermal and subsequently recolonised, further contributing to the lack of endemism. The Adelbert upland avifauna is more closely related, in presence / absence and taxonomic relationships, to that of the nearby Huon Mts. to the east than to the avifauna of the more distant North Coastal Range to the west. That suggests why the Adelberts support S. bakeri as such a distinctive endemic but the rest of their avifauna is undifferentiated: Sericulus is the only upland superspecies of north New Guinea that reaches its eastern distributional limit in the Adelberts; and its low elevational floor permitted it, but not higher-elevation species, to survive upwards shifts in range during the hypsithermal. An appendix summarises all 235 species recorded from the Adelberts, our observations of their elevational range and abundance, and their names in two local mountain languages of the Adelberts.