In "Birders: Tales of a Tribe", author Mark Cocker erroneously describes the committee as a "statutory vetting body" (i.e. one established by an Act of Parliament). BBRC's constitution states that it "has no automatic or legal expectation that birders submit records". The committee does not assess records of birds from Ireland; that task is carried out by the Irish Rare Birds Committee, which publishes its decisions in "Irish Birds". Records of IRBC-assessed rarities were included in BBRC annual reports for many years, although this ceased in 2002 at the request of IRBC. Although the British Ornithologists' Union Records Committee does not regard records from the Isle of Man as contributing towards their British list, BBRC does include records from there in its totals. BBRC has had an ongoing sponsor in the German optical equipment manufacturer Carl Zeiss since 1983. 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). The committee keeps the list of species it considers under review and, from time to time, makes changes. These are usually because species have increased in frequency and no longer meet the numerical criteria for inclusion. A species is removed if it has more than 150 records in the preceding ten years, with ten or more in at least eight of those years. Different criteria were used in a review in 2006 (see below). There have been three major "purges" of species since the committee's formation. The first was soon after the committee's formation, in 1963, when 16 species were removed: red-crested pochard, snow goose, pectoral sandpiper, Mediterranean gull, Sabine's gull, melodious warbler, icterine warbler, yellow-browed warbler, northern goshawk, golden eagle, red kite, Kentish plover, crested tit, bearded tit, marsh warbler and Dartford warbler.