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"The site is known or thought to hold a significant component of the group of species whose distributions are concentrated in a sub-biome."

This criterion is meant to assure that species are protected in the core of their range, regardless of their current threat. The criterion is based on the principle that we bear a responsibility to assure the continued support of species in the core areas of their range.

This criterion applies to groups of bird species with largely shared distributions, which are concentrated within a particular sub-biome. Many of these assemblages are found in large areas of relatively intact and continuous habitat where delimiting IBAs may be particularly difficult. Seabirds are excluded from these lists because their distributions are thought to be influenced by different factors than those affecting terrestrial species, and their conservation is covered through the application of other criteria.

In the U.S., sub-biomes are represented by Bird Conservation Regions (BCRs), the geographic units recognized by the North American Bird Conservation Initiative for conservation planning. As defined by the U.S. IBA Committee, species which are BCR-restricted under the B3 criterion are those with least 30% of the population concentrated within one BCR. Note that the list does not include Hawaiian species, as these species ranges cover too small an area.

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