An EBA is formed where the distributions of two or more such restricted-range species overlap.
[1] Using this guideline, 218 EBAs were identified when Birdlife International established its Biodiversity project in 1987.
Although they cover less than 5% of the world's land surface, their biological richness makes them high priorities for ecosystem conservation.
It is defined as a landbird (i.e. not a seabird) species which is estimated to have had a breeding range of not more than 50,000 km2 since 1800.
It does not include birds which, although they meet the range criterion today, were historically (since 1800) more widespread.