Understanding how species respond to climate and land use change is ofprime importance for biodiversity conservation. However, studies using histori-cal, empirical data to understand species responses to climate change andanthropogenic habitat disturbance remain scarce, especially for tropical moun-tain ecosystems. Here, we compare historical to recent mist net data to studychanges in abundance of forest understorey birds along two elevational gradi-ents. We investigate patterns of understorey bird abundance along eleva-tional and disturbance gradients on two slopes of Mt. Kilimanjaro between1991 and 2011. To understand species responses to climate change and distur-bance, we use a trait-based approach. Specifically, we test whether and howspecies' traits (temperature, habitat and dietary preferences) affect speciesresponses to environmental change. We found that over the 20-year time interval, mean minimum temper-ature increased across all sites along the elevational gradient, while levels ofhabitat disturbance remained similar. Mean abundance of understorey bird spe-cies increased significantly, especially at higher elevations and on the southernslope. Temporal increases were not related to species' temperature preferencesand were detected for habitat generalists as well as herbivorous and omnivorousspecies, but not for forest specialists and insectivores. Our results suggest subtle effects of climate change on birdabundances which would not have been detectable if we had examined eleva-tional range shifts or changes in species richness. Both warm- and cold-adaptedspecies appear to have benefited from increases in minimum temperatures athigh elevations. This was not the case for forest specialists and insectivoreswhich remained stable between sampling periods. This demonstrates that differ-ent groups of birds vary in their response to ongoing climatic changes. Ourresults underline the importance of tropical mountain ecosystems not only asglobal biodiversity hotspots, but also as safe havens for biodiversity in the faceof global climate change.
