Different zoogeographic realms have varying diversity of bird species.

The Palearctic, consisting of Europe, Africa north of the Sahara, & almost all of Asia, is the largest area and may be where birds originated. It contains about 1025 species and most are migratory.

The Nearctic consists of North America (north of the tropics) & Greenland. It includes over 1 billion acres of boreal forest, the deciduous forests of the eastern U.S. and southeastern Canada, the grasslands of the midwestern U.S. and Canada, and the deserts of the southwestern U.S. and northern Mexico. The Nearctic has low species diversity, with only about 750 species of breeding birds.

The Neotropical is made up of South America, Central America, lowlands of Mexico, & the West Indies. About 70% of land mass is low, well-watered, & tropical. South America is sometimes called “the bird continent,” and the Neotropical has easily the greatest bird diversity of all realms with about 3700 species (or more than 1/3 of all bird species) and about 31 endemic families – more than twice that of any other realm.

The Ethiopian (or Afrotropical) realm consists of Africa south of the Sahara, southern Arabia, & Madagascar. There is a large proportion of desert, grassland, and savanna habitats. It is home to about 1800 species of birds

The Oriental (or Indomalayan) realm includes India, southeast Asia (Malay Archipelago), Java, Borneo, & the Phillipines. It is home to about 2400 species of birds. 66 families of land and fresh-water birds reside in this region, but only one, the Irenidae (Leafbirds & Fairy Bluebirds), is endemic.

The Australasian realm is made up of Australia, New Zealand, Papua New Guinea, & several small islands in the area. It is generally a dry region, only 4.5% rainforest. About 70% of Australia is desert, and in this area only 17 species of birds occur (3% of Australia’s avifauna). There are about 1600 species in the Australian region, and about 64 families of terrestrial and freshwater birds.