From the Am Nat press release:
Why do related tropical montane birds live at different elevations? Competition on secondary contact drives divergence.
Whether bat or butterfly, beetle or bird, tropical montane species tend to inhabit narrow swaths of elevation. This makes tropical mountains an ideal destination for biodiversity enthusiasts—after watching a dozen hummingbird species in the Amazonian foothills, you can hop in a car, drive half an hour uphill to the mossy Andean cloud forests and find a new set of a dozen or so brightly-colored hummers to admire. Thus, while species richness is high in hot lowland and chilly montane forests, very few species are found in both. This explains in part why tropical mountains are the hottest of biodiversity hotspots, yet the evolutionary processes that lead closely related species to specialize on different elevational zones remain murky.
Ben Freeman, a graduate student at Cornell University, has investigated the evolution of elevational distributions in the tropical montane birds of the Neotropics, New Guinea, and the Himalayas. He finds that elevational divergence between avian sister species is predicted only by whether they live on the same mountain slope, where they can interact ecologically. Whether speciation has occurred recently or in the distant past, sister species that live on the same mountain slope tend to live at different elevations, while those in distinct regions tend to have similar elevational distributions—to “do the same elevational thing in different places.” Despite the enormous differences between the birds of the Andes, New Guinea, and the Himalayas, Freeman finds the same pattern in each region. This repeated pattern suggests that ecological interactions during secondary contact, such as interspecific competition, are responsible for the evolution of elevational divergence