This chapter summaries what is known about population trends of mountain birds, especially in Europe and North America. A European mountain bird indicator, which summaries the population trends of 44 alpine species, suggests an overall slightly increasing mountain bird population during 2002–2020. Regional North American indicators, based on up to seven alpine species showed either stable or declining trends during 1968–2020. In European mountains, cold-dwelling species had on average less favourable regional population trends than warm-dwelling species, and long-distance migrants tended to have more negative trends than short-distance migrants and residents. There were also spatial differences in trends of the indicators in Europe: mountain birds in general increased in the Alps but decreased in the UK. A comparison between two European breeding bird atlases showed that the distribution area of mountain birds has generally decreased since the 1980s, and mountain specialists have lost more of their range than mountain generalists. Monitoring alpine species presents many challenges which has led to poor coverage in surveys even in areas with well organised bird monitoring programmes at low elevation. The necessary future improvements needed for successful bird population monitoring in mountain areas will, in many instances, require strong financial support.

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.