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.