Each species on the British List is placed in one or more categories denoting its status on the List.
In 1997, categorisation was revised to assist protection under national wildlife legislation, especially of naturalised species. Category C was expanded to allow species with different histories of introduction and naturalisation to be distinguished; Category D (not included in the List totals) was reduced in scope, and a Category E (not included in this list) was introduced to enable local and national recorders to monitor escaped species.
Species recorded in an apparently natural state at least once since 1 January 1950.
Species recorded in an apparently natural state at least once between 1 January 1800 and 31 December 1949, but have not been recorded subsequently.
Species that, although introduced, now derive from the resulting self-sustaining populations.
- 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. possibly some Ruddy Shelducks Tadorna ferruginea occurring in Britain. There are currently no species in category C5.
- C6 – Former naturalized species - species formerly placed in C1 whose naturalized populations are either no longer self-sustaining or are considered extinct, e.g. Lady Amherst’s Pheasant Chrysolophus amherstiae.
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.
Species 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 F (list being compiled)
Records of bird species recorded before 1800.
- F1 – Records of bird species recorded between c. 18000 BP (before present) to 1800
- F1.1 – 1500 - 1800 – Post Medieval
- F1.2 – 1100 - 1500 – Medieval
- F1.3 – 400 - 1100 – Anglo-Norman
- F1.4 – 0 - 400 – Roman
- F1.5 – c. 3500 BP - 0 – Iron Age
- F1.6 – c. 45000 - c. 3500 – BP Bronze Age
- F1.7 – c. 6000 - c. 4500 – BP Neolithic
- F1.8 – c. 11000 - c. 6000 – BP Mezolithic
- F1.9 – c. 18000 - c. 11000 -BP Late Glacia
- F2 – Records of bird species recorded earlier than 16000 BP, back to c.700,000 BP
- F3 – Specimens or records of uncertain species or date
Each of the above can be further subdivided to denote (a) fossil or bone specimen and (b) documentary only records, e.g.
F1.1a = A fossil or bone specimen record from between 16000 BP to 1800 AD
F1.1b = A documentary only record from between 16000 BP to 1800 AD
A species is usually placed in only one category, but some are placed in multiple categories, for example, those species occurring in Category A which now have naturalised populations (e.g. Red Kite).
Category F is currently under construction.
The British List comprises only those species in Categories A, B and C.
The newly created Category C6 recognises that some previously established naturalized introductions to Britain have declined (and others may do so in the future) to a level that is no longer self-sustaining, and which will ultimately lead to extinction. Further releases of such non-native species are prohibited under Section 14 of the Wildlife & Countryside Act 1981.