Criteria

Seven quantitative criteria were used to assess the population status of each species and place it onto the red, amber or green list. These criteria are listed below. The review excluded species that are not native to the UK¹ and those that occur irregularly as vagrants or scarce migrants.

Global conservation status
Species assessed as Globally Threatened using IUCN¹¹ criteria were placed on the red list.

Recent decline
Species whose breeding or non-breeding population declined, or range contracted, rapidly (by more than 50%) or moderately (by between 25 and 49%) over the last 25 years were placed on the red and amber lists respectively.

Historical decline
Species whose populations declined severely between 1800 and 1995 were placed on the red list, except for those that have recovered substantially (more than doubled) in the last 25 years, which were amber-listed¹². In earlier assessments, all species showing a serious historical decline were red-listed, but in this assessment the success of recent conservation action has been recognised by moving recovering species to the amber list.

European conservation status
Species whose population status is unfavourable in Europe (but which are not Globally Threatened) were placed on the amber list.

Rare breeders
Species with a mean population size of 1-300 pairs breeding annually over the last five years were placed on the amber list. If a full census was carried out in a single year, the result of this was used instead of a five-year mean.

Localised species
Species for which 50% or more of the breeding or non-breeding population occurs at 10 or fewer sites were placed on the amber list. This criterion was used because a species whose population is confined to a few sites faces a greater threat from chance events than one whose population is widespread. The sites considered were either Important Bird Areas (identified by BirdLife International) or Special Protection Areas (designated under the European Union's Directive on the Conservation of Wild Birds).

International importance
Species with 20% or more of their European population breeding in the UK were placed on the amber list, as were non-breeding wildfowl with 20% or more of their northwest European population occurring in the UK and non-breeding waders with 20% or more of their East Atlantic Flyway population occurring in the UK. This criterion is different from the others as it is a measure of the UK's responsibility for each species rather than the extent to which species are threatened.

Red list species are those that are Globally Threatened according to IUCN criteria; those whose population or range has declined rapidly in recent years; and those that have declined historically and not shown a substantial recent recovery.

Amber list species are those with an unfavourable conservation status in Europe; those whose population or range has declined moderately in recent years; those whose population has declined historically but made a substantial recent recovery; rare breeders; and those with internationally important or localised populations.

Species that fulfil none of the criteria are green-listed.