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.

We review evidence for whether and why mountain birds are potentially more vulnerable to climate change than birds at low elevation, given current knowledge of the avian limits to physiological and environmental conditions. The observed impacts of climate change on bird populations, communities and distributions in mountain environments are limited.  We summarise these responses and provide an assessment of the main mechanisms that might underpin those changes, such as changes in climate (temperature, precipitation, snow cover extent), and biotic drivers (phenology/food mismatch, prey/predator populations, shifting treelines). The potential future impacts of climate anticipated in mountain environments, considering the vulnerability of montane species and the likelihood of increasing upslope colonisation by species that currently occupy lower elevations, are considered, including a comparison between the observed and projected impacts where available.  Finally, we discuss potential conservation actions for mountain birds in the face of climate change.  Are there ways that the management of these habitats can be altered to either help vulnerable species resist negative impacts, or to facilitate more positive changes in response to climate change? We conclude by summarising anticipated avifaunal community composition and function in these mountain environments by the end of the century.

Alpine birds face many challenges to live in habitats characterized by low temperatures, high winds, short growing seasons and delayed breeding schedules. Breeding in alpine environments is always a race against time due to the shortened egg laying period and frequent storms that may result in delayed development or reproductive failure. Since daily temperatures in the alpine zone can range from below freezing to >450C, developing embryos may require cooling as well as heating to maintain homeothermy. To cope with such conditions, birds breeding in alpine habitats have developed physiological, morphological and behavioural adaptations, and have adopted a slower lifestyle where they may produce fewer offspring each year compared to populations at low elevations, but may live longer. In the northern hemisphere, only a few birds live exclusively in the alpine zone, with many mountain species breeding in both alpine and lower elevation habitats, while in the Southern Andes, most alpine birds breed exclusively above the treeline. In summary, there may be high ecological costs to living in open habitats at high elevations. However, alpine birds likely experience lower levels of interspecific competition, habitat degradation and parasites and diseases than birds living at lower elevations.

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.

Tropical mountain regions are characterized by complex and mostly resident avifaunas with many small-range species and a high turnover of species across mountain ranges. On a global scale, tropical mountains show an over-representation of both recently-diverged and ancient species, making them both cradles and museums of biodiversity. Tropical mountains are characterized by slight seasonality, and local habitat gradients can be maintained over a long time. The highest levels of diversification and local endemism are found in the tree-line zone. However, a few avian families also diversified in the alpine zone. Here, many species forage by probing for invertebrates in the ground and in matted vegetation, and many species also exploit carbohydrate foods, including nectar. In an environment with few large insects, co-adapted networks of nectarivorous birds and plants play an important role. There is little published evidence of avian population changes that can be related to recent global warming, as the night-time freezing conditions in open landscapes make it difficult for arboreal vegetation to expand upslope. As glaciers melt, we should expect changes in the rich avifauna of the many periglacial Andean wetlands. Re-visits to well-documented study sites in tropical mountains are now needed to evaluate the amount of change.

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.