Birds on islands in the sky: Origin of the montane avifauna of Northern Melanesia.

Biogeographers have long been fascinated by the disjunct distributions of species stranded on mountaintops. This paper analyzes, for the montane bird populations of Northern Melanesian islands, how many such populations there are, why they are restricted to mountains, and how they dispersed to mountains. The number of populations increases with island elevation and with montane area, and decreases with lowland area, exemplifying the problem of continental species diversity. Most species with montane populations on some island(s) have sea-level populations on some other island(s). These altitudinal niche shifts can be variously related to interisland differences either in altitudinal distribution of area or else in competitive pressure in the lowlands or mountains. Restriction of Northern Melanesian bird populations to mountains is more often due to lowland competitors than to inability to survive under the physical conditions of the lowlands. Of four possible mechanisms for the origin of a montane population (referred to as jumping, land-bridge crossing, trickling, and push-pull shifts), only the first and last have been significant for Northern Melanesian birds.