Founded in 1858, the BOU is one of the world’s oldest and most respected ornithological organisations with an international membership stretching across all continents. One of its roles is to maintain the British List and in doing so it is necessary to maintain data on species that although they have occurred in a wild state do not fit the criteria for admittance to that list.
Birds on category E are usually referred to as "escapes". These birds have been seen in the wild in Britain but have escaped or been released from a zoo or private collection or they were deliberately introduced or they got here by hitching a ride on a ship. Ultimately they are recaptured or they die without establishing a wild population. These species do not form part of the British List (unless they also occur in category A, B or C).
Please Note: Some category E species e.g. Bewick's Swan Cygnus columbianus are also on the British List, i.e. some individuals occur here in a wild state but there are also escapees, and such species are not looked at in detail on this website. The British List page identifies birds that hve occurred in a truly wild state but which are also in category E.
"E. Species that have been recorded as introductions, human-assisted transportees or escapees from captivity, and whose breeding populations (if any) are thought not to be self-sustaining. Species in Category E that have bred in the wild in Britain are designated as E*. Category E species form no part of the British List (unless already included within Categories A, B or C)."
Category D is something of a "holding category" There are relatively few birds in this group; they have not definitely escaped but there is reasonable doubt that they occurred here naturally. The Greater Flamingo Phoenicopterus roseus is an example; there is a thriving population in the south of France and it is possible that they fly to the south coast of Britain but on the other hand, there are a great many in zoos around the country and they do undoubtedly, escape on occasions. If a proven wild bird was recorded here, then Greater Flamingo would also become a member of category A. (It would remain on category E as well because most birds would still be considered to be "escapes"). On the other hand, the flightless Common Ostrich, which is on category E through it having escaped from captivity, cannot conceivably ever make it to category A. (Or C for that matter as such a potentially dangerous bird will never be allowed to establish a wild breeding population in Britain).
"D. Species that would otherwise appear in Category A except that there is reasonable doubt that they have ever occurred in a natural state. Species placed in Category D only, form no part of the British List and are not included in the species totals."
There are over 600 species and over 700 taxa (i.e. including sub-species) on the 'British List'. These are birds that have always been here or have occurred here naturally, perhaps by being blown off-course during migration or through adventurous travelling during the lazy days of autumn or through overshooting their spring migration during spring passage from Africa. Others are on the British List because they have self-sustaining populations but were originally brought here by man. If they have occurred naturally since 1950, they are on category A. If they occurred naturally between 1800 and 1950 (but not since) they are in category B and if they were introduced, but now exist in an apparantly wild state without further assistance (i.e. are "naturalised") they are in category C. Species in categories A, B and C form the "British List"; these are the ones that birdwatchers tick off and that twitchers travel so extensively to see.
"A. Species that have been recorded in an apparently natural state at least once since 1 January 1950.
B. Species that were recorded in an apparently natural state at least once between 1 January 1800 and 31 December 1949, but have not been recorded subsequently.
C. Species that, although introduced, now derive from the resulting self-sustaining populations."
Category C is subdivided as follows:
"C1 Naturalized introduced species – species that have occurred only as a result of introduction, e.g. Egyptian Goose Alopochen aegyptiacus.
C2 Naturalized established species – species with established populations resulting from introduction by Man, but which also occur in an apparently natural state, e.g. Greylag Goose Anser anser.
C3 Naturalized re-established species – species with populations successfully re-established by Man in areas of former occurrence, e.g. Red Kite Milvus milvus.
C4 Naturalized feral species – domesticated species with populations established in the wild, e.g. Rock Pigeon (Dove)/Feral Pigeon Columba livia.
C5 Vagrant naturalized species – species from established naturalized populations abroad, e.g. Sacred Ibis Threskiornis aethiopicus from the naturalized French populations. There are currently no species in category C5.
C6 Former naturalized species – species formerly placed in C1 whose naturalized populations either are no longer self-sustaining or are considered extinct, e.g. Lady Amherst's Pheasant Chrysolophus amherstiae.".
There is also a category F, which is under development and will document those species that were recorded before 1850, right back to pre-historic times and known only from the fossil record.