The mountain treeline forms the ecotone between the forest and the grass/shrub-heath dominated alpine zone and is often a zone of relatively high diversity. We first assess the universality of a peak in bird diversity at the treeline across studies and regions. Defining a ‘treeline’ bird is challenging, as this zone is often a mix of habitats, thus we determine whether any species can be classified as ‘treeline specialists’. We then compare bird communities of different mountain ranges and types of treeline: climate-limited, and those limited by disturbance, including of both natural (e.g. avalanches, grazing by wild herbivores) and anthropogenic (livestock grazing) origin. Furthermore, we assess the extent to which there are commonalities in broad traits (e.g. migratory strategy, foraging strategy, nesting substrate), primary origin (e.g. montane or alpine) and taxonomy (e.g. proportion of passerines vs non-passerines) across different treeline communities. We then review the determinants of reproductive success of treeline birds, the importance of vegetation structure, and the impact of grazing of both domestic and wild animals in maintaining the habitat mosaic. Finally, we consider how key threats to treeline bird communities might be confronted through conservation strategies.