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. Records of the last eight species had been considered only outside their "normal" British range (e.g. crested tits outside Scotland, and red kites outside Wales). The second was in 1982, when ten further species were removed: Cory's shearwater, purple heron, white stork, buff-breasted sandpiper, Richard's pipit, tawny pipit, Savi's warbler, aquatic warbler, serin and common rosefinch. Savi's warbler was re-admitted to the committee's list in 1998, due to declining numbers of occurrences. Thirdly, in 2006, a further 17 species were dropped from the list: ferruginous duck, Wilson’s petrel, great white egret, black kite, red-footed falcon, American golden plover, white-rumped sandpiper, white-winged black tern, alpine swift, red-rumped swallow, red-throated pipit, subalpine warbler, greenish warbler, dusky warbler, Radde's warbler, Arctic redpoll and rustic bunting. Their selection was based on different criteria from those previously used: more than 200 records in Britain ever, and more than 100 occasions in the last decade.