- Scarce - Species that are scarce for the county
- Description (EBSIP) - Species that are extremely rare for the county and for which a full description is required by the Essex Birdwatching Society Identification Panel
- Description (BBRC) - Species of national significance for which a full description is required by the British Birds Rarities Committee
- Category A - Species recorded in an apparently natural state since at least 1st Jan 1950
- Category B - Species recorded in an apparently natural state at least once between 1st Jan 1800 and 31st Dec 1949 but not subsequent to that
- Category C - Species that, although introduced, now derive from the resulting self-sustaining populations
- Category D - Species that would otherwise appear in Category A except that there is reasonable doubt that they have ever occurred in a natural state. These do not form part of the British List.
- Category E - Species recorded as introductions, human assisted transportees or escapees from captivity, and whose breeding populations (if any) are thought not to be self-sustaining. These do not form part of the British List (unless they are included in Categories A, B or C)
- Category F - Records of Bird species recorded before 1800.