Over 580 bird species have been recorded in Britain; some 250 have regular breeding or wintering populations, or are common migrants, and a further 50 are "scarce migrants". The BBRC assesses the remaining species. A list of the species the committee assesses (known as "British Birds rarities", or "BB rarities" for short) is on its website, although the list has not been updated to account for the most recent changes. The committee generally considers only the records of species rare enough to meet its criteria for inclusion on the BBRC rarities list, based on a numerical threshold. In addition to considering full species, records of some rare subspecies are considered and, in a few cases, the committee considers indeterminate records (birds that belong to a pair or group of rare species that are particularly difficult to separate, such as frigatebirds or dowitchers).