mature individuals

When the IUCN measures how close a species sits to extinction, it does not use the total number of animals alive. It uses the number of mature individuals: the individuals known, estimated or inferred to be capable of reproduction. The definition sits in the IUCN Red List Categories and Criteria, the rulebook every assessment follows, and it exists because extinction risk depends on how many animals can produce the next generation rather than on how many are present.

The rules are deliberately conservative. Animals that will never produce young are not counted, even when they are alive and healthy. Where a population swings in size from year to year, assessors are told to use the lower estimate. Reintroduced animals count only once they have produced viable young of their own. For many species the working figure comes from applying a measured ratio of breeding adults to a headcount, so a census of several thousand birds can translate into a published population that is much smaller.