Birds are found worldwide in all countries and major habitats, with some being particularly important

Birds occur in every country in the world and in virtually every habitat, from the lowest deserts to the highest mountains. Some generalist species may utilise several habitats, but many specialists are confined to just one. Forests are the most species rich habitat, supporting 78% of all bird species. The global distribution of these habitats, alongside other fundamental biogeographic factors, drives patterns of bird diversity, with tropical countries (especially in South America) supporting the highest species richness.

A total of over 11,000 different species of birds are recognised by BirdLife International, the majority (c.80%) occurring in continental regions, the remainder on islands. Birds occupy a huge variety of habitats and are found at the extremes of latitude and land elevation.

Although some birds occur in two or more habitats, many specialist species are confined to just one. Grasslands, savanna and inland wetlands are all important habitats for birds, each supporting about 20% of extant species (see figure 1), while shrublands support 41% of birds. Around 49% are found in ‘artificial’ terrestrial habitats; those that have been modified by humans such as agricultural land, but by far the most significant habitat is forest, supporting 78% of all species (BirdLife International 2023). Around a third of these forest-dwelling species are highly dependent on this type of habitat. Birds are found in all forest types, from subantarctic woodland to equatorial rainforest (see figure 2). The most important types are tropical/subtropical lowland and montane moist forest, which support 67% and 49% of forest species respectively, with tropical/subtropical dry forest supporting 25% (BirdLife International 2023).

The distribution of these habitats globally, among other biogeographic factors, influences the distribution of birds. The different biogeographic realms vary substantially in terms of the numbers and types of bird species they hold (see figure 3). By far the richest is the Neotropical realm, which holds c.36% of all known landbird species. This is followed by the Afrotropical (c.21%), Indomalayan (c.18%), Australasian (c.17%), and then the Palearctic (c.10%), Nearctic (c.8%) and Oceanic (c.2%) realms (BirdLife International 2023). Although they have relatively few species in total, the Pacific islands in the Oceanic region are unusually rich for their size; together they hold 20 times more species per unit area than South America, the richest of the continents (Newton 2003). Country by country, the richest territories for avian diversity are Colombia, Peru, Brazil, Indonesia and Ecuador (each with more than 1,600 species), followed by Bolivia, Venezuela, China, India, the Democratic Republic of Congo, Mexico, Kenya, Tanzania, Myanmar and Argentina (all around 1,000 or more; BirdLife International 2023).

There is much debate over what factors have been important in driving global patterns in biological diversity. The existence of big geographic differences in bird species diversity is thought to result from the differing conditions experienced over evolutionary time. Particularly influential is the variety (and area) of different habitats present. Tropical forests are especially rich in species; hence the particularly high avian diversity found in the equatorial regions. Other major influences include physical barriers such as impassable oceans and mountain ranges, climatic events such as the recent glacial cycles, biotic constraints such as natural enemies and competing species and, more recently, expanding and pervasive human impacts. The distributions of other taxa are less well-known than those of birds, but they are also determined by these fundamental biogeographic factors. This makes birds a useful starting point for mapping broad-scale patterns in species richness and endemism.