High mountains cover an estimated 25% of the global land surface, but harbour almost 50% of terrestrial biodiversity hot-spots and about one-third of terrestrial biodiversity globally. Thus, it is concerning that relatively little research has been conducted on birds in high mountains, especially for tropical mountain birds. We identified 10 major knowledge gaps arising from the reviews in our nine previous chapters, including the urgent need for information on avian diversity and population and community ecology, especially in under-studied mountains of the Global South, avian responses to climate change and other stressors, mountains as refugia from habitat and climate change, and the role of protected areas to function as biodiversity reservoirs. We propose a set of priorities for ecological and conservation research and management that will help to ensure persistence of birds in high mountain ecosystems. Maintaining and restoring mountain biodiversity is important from ecological, evolutionary and cultural points of view. We recommend investing in research to safeguard the critical ecological, social and economic values of mountain systems into the future. Strong support is needed from the scientific community, citizen scientists, policy makers, politicians and local communities to fulfill our priorities for the conservation of mountains and mountain birds.

There are many definitions of what is a ‘mountain’ and what is a ‘mountain bird’. In this chapter, we first assess these different definitions, and then clearly outline our rationale for choosing to define a mountain bird as bird species where at least some populations of the species somewhere in their distribution spend at least one critical stage of their life cycle above treeline. We then provide an overview of the importance of mountains to biodiversity, and compare knowledge on mountain birds to lowland ecosystems. Zonation is an important aspect of mountain ecology – we review the evidence for consistent patterns in bird richness and diversity across elevation gradients, and consider the different hypotheses that might explain these patterns. Additionally, we consider variation along the elevation gradient in some general species characteristics and the extent to which these trends vary geographically. Furthermore, we give an overview of how mountain bird communities vary seasonally, in particular considering different dispersal and migration strategies, and the extent to which the prevalence of these strategies varies according to different regions. Finally, we summarise the history of human interventions in mountains and their impacts on bird communities from pre-history until the start of the mechanized age.

Mountain areas have faced a rapid increase in human activities over recent decades, often leading to habitat loss or degradation. The impacts of these activities can affect bird species both directly (e.g., by altering habitat characteristics, impacting migration or disturbing breeding or wintering grounds), and indirectly by inducing physiological responses. We summarize the human activities that take place at high elevation and provide examples of species that are known to be impacted. Hiking and winter sports in particular are common in many mountain regions and there is growing evidence of a range of impacts on year-round resident mountain birds and their food resources. Increasing evidence also suggests that use of, and dependence on, human-derived foods around human settlements affects the trophic ecology of high-altitude birds. Hunting mountain birds is common place in many areas, and we review the evidence that hunting activity, including illegal persecution, has had impacts at the population level. Finally, we assess how direct disturbance and habitat alteration due to renewable energy developments (i.e. wind turbines and hydropower) are affecting mountain bird communities. There are many unknown impacts of human disturbance and we highlight missing information on specific topics that should be investigated in future research.

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.