A disproportionately high number of threatened birds, almost half, occur on islands, particularly oceanic islands far from land. This pattern is no surprise because island species typically evolved in isolation and are maladapted to deal with recently emerging, human driven, threats.
Most of the world’s bird species are found on the continents; only c.17% are restricted to islands (Johnson and Stattersfield 1990). However, islands have disproportionately high numbers of threatened species: almost equal numbers of threatened birds are found on continents (613 species), and islands (583 species), with few (30 species, 2.5%) shared between them (BirdLife International 2008). Oceanic islands are more important for threatened birds, supporting 431 species, than islands located on continental shelves close to mainland, which support just 174. Some small islands harbour exceptionally high numbers of threatened birds, with, for example, 18 on Mauritius and 16 on the Northern Mariana Islands (to USA). The Pacific island of Nauru—one of the world’s smallest states, with an area of just 18 km2—supports two threatened birds, one of which (Nauru Reed-warbler Acrocephalus rehsei) is found nowhere else (BirdLife International 2008). These patterns are not unexpected. Species on oceanic islands are often particularly susceptible to human driven changes; especially the impacts of introduced predators, having evolved in isolation for many thousands of years.
Compiled 2004, updated 2008
BirdLife International (2008) Many threatened birds are restricted to small islands. Presented as part of the BirdLife State of the world's birds website. Available from: http://birdlife.org/datazone/sowb/casestudy/173. Checked: 30/11/2015