Terms and definitions

A joint initiative of biodiversity organisations from around the world working to prevent extinctions by promoting the identification, safeguarding and effective conservation of key sites that are the last remaining refuges of one or more Endangered or Critically Endangered species. Find out more at https://zeroextinction.org/

A scaled metric that represents the area within the Extent of Occurrence (EOO) that is occupied by a taxon, excluding cases of vagrancy. It reflects the fact that a species may not occur throughout the entirety of its EOO—there may be patches of unsuitable or unoccupied habitat. In the case of migratory species, estimates of AOO for application to IUCN Red List assessments are based on the minimum of the breeding or non-breeding (wintering) areas, but not both, because such species are dependent on both areas, and the bulk of the population is found in only one of these areas at any time.

Conservation actions in place and needed are recorded as part of the IUCN Red List assessment for each species, and in some site assessments. These are coded using the IUCN Conservation Actions Classification Scheme and the IUCN Research Needed Classification Scheme.

A recent, current or projected future decline (which may be smooth, irregular or sporadic), for example in population size or distribution, which is liable to continue unless remedial measures are taken.

A species that is only found in a specific geographical area (usually a country), and nowhere else. A “breeding endemic” is a species that breeds only in a single country, but may occur in other countries during other stages its annual cycle.

A parameter used to measure the spatial spread of the areas currently occupied by a species, excluding cases of vagrancy. It is calculated by drawing a Minimum Convex Polygon around the species’ mapped range. In the case of migratory species, estimates of EOO for application to IUCN Red List assessments are based on the minimum of the breeding or non-breeding (wintering) areas, but not both, because such species are dependent on both areas, and the bulk of the population is found in only one of these areas at any time.

Fluctuations in population size or distribution area are classed as extreme if they vary widely, rapidly and frequently, typically with a variation greater than one order of magnitude (i.e. a tenfold increase or decrease).

Dependence on forest for each species is categorised as:

High: Forest specialists; characteristic of the interior of undisturbed forest; may persist in secondary forest and forest patches if their particular ecological requirements are met, but where they do occur away from the interior, they are usually less common; rarely seen in non-forest habitats; breeding is almost invariably within forest.

Medium: Forest generalists; may occur in undisturbed forest but also regularly found in forest strips, edges and gaps; likely to be commoner in such situations and in secondary forest than in the interior of intact forest; breeding is typically within forest.

Low: Often recorded in forest, but not dependent on it; almost always more common in non-forest habitats, where most likely to breed

Unknown: Occurs or probably occurs in forest, but dependency on it is unknown, but could be high

Does not normally occur in forest

The average age of parents of the current cohort (i.e. newborn individuals in the population). Generation length therefore reflects the turnover rate of breeding individuals in a population. It is greater than the age at first breeding and less than the age of the oldest breeding individual, except in taxa that breed only once.

Species’ generation lengths are necessary for scaling population declines for assessing extinction risk, and are required for IUCN Red List assessments. An approach to systematically estimate generation lengths for all bird species was published in 2020 (Bird et al. 2020), where compiled information on life history and trait data was used to impute missing life history data using Generalized Linear Mixed Models. Generation lengths for all bird species were then calculated using the modelled values of age at first breeding (F), maximum longevity (L) and annual adult survival (S).

We continue to update the life history and trait database underpinning this work, and periodically repeat the methodology to generate improved estimates of generation length for use in each comprehensive Red List reassessment cycle. The most recent results are from August 2025, after an extensive update to the underlying data set. For some species, a generation length based on life-history tables has been calculated, which is used in preference to the value from the model-based methodology.

Over the next few years, BirdLife will be aligning its taxonomy to AviList. Updated values for newly 'split', 'lumped' or 'revised' species will be provided each year to match the updated BirdLife taxonomic checklist during this period of transition.

A species may move from one IUCN Red List Category to another. For example, a species may be moved to a lower category of extinction risk because the main threats are no longer present, or because conservation actions have successfully improved the status of the species. A species may be moved to a higher category of extinction risk if the main threats have continued unabated, have increased, or new threats have developed causing the status of the species to deteriorate. Non-genuine reasons for a species to change Red List Category include new information becoming available since the last assessment, taxonomic revisions, errors being discovered in the previous assessment, and changes to IUCN Red List Criteria since the last assessment.

Habitats utilised by species or present in Important Bird and Biodiversity Areas (IBAs)/Key Biodiversity Areas (KBAs) are coded using the IUCN Habitats Classification Scheme. The scheme uses a hierarchical structure to describe terrestrial, wetland and marine habitats.

For each species, habitat suitability is coded as:

Suitable: the species occurs in the habitat regularly or frequently. A suitable habitat may also be coded as being of Major Importance. This indicates that it is important for the survival of the species, either because it has an absolute requirement for the habitat at some point in its lifecycle (e.g. for breeding or as a critical food source), or it is the primary habitat (or one of two primary habitats) within which the species usually occurs or within which most individuals occur.

Marginal: The species occurs in the habitat only irregularly or infrequently, or only a small proportion of individuals are found in the habitat.

Unknown: The habitat is of unknown importance to the species

For some species, seasonality of habitat occurrence is important (e.g. migratory species) and this may also be recorded against each habitat.

For the purposes of IUCN Red List assessments, a location is defined as a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of a species. Where a species is affected by more than one threatening event, location is defined by considering the most serious plausible threat. Fewer locations means that larger parts of a species' range are subject to the same threat, resulting in less risk-spreading, more correlated (synchronised) declines due to threats, and therefore greater extinction risk.

Individuals known, estimated or inferred to be capable of producing offspring. This does not include juveniles, senescent individuals, suppressed individuals, or individuals in subpopulations whose densities are too low for fertilization to occur.

Movement patterns of each species are categorised as:

Full migrant: A substantial proportion of the global or regional population makes regular or seasonal cyclical movements beyond the breeding range, with predictable timing and destinations. This includes species that may be migratory only in part of their range or part of their population, short-distance migrants, and species that occasionally may respond to unusual conditions in a semi-nomadic way. Migratory species may require conservation action (at specific sites and/or beyond sites) along their migration routes.

Altitudinal migrant: Regularly/seasonally makes cyclical movements to higher/lower elevations with predictable timing and destinations. Partial altitudinal migrants might not be best conserved at the site scale alone, if individual sites do not encompass the full altitudinal range of the species.

Nomadic: Moves in response to resources that are sporadic in time and distribution. Nomadic species may congregate, but not predictably in terms of location and timing. Nomadic species have evolved to move but usually cannot be conserved at the site scale alone.
Not a migrant: Not nomadic or migratory (full or altitudinal) as defined above.

Unknown: Not known if the taxon has any movement patterns.

For each country that a species occurs in, or has occurred at some point in the past, it is assigned a presence, origin and seasonality code. Further details on these codes can be found in the Mapping Standards and Data Quality for the IUCN Red List Spatial Data.

Presence

Extant: The species is known or thought very likely to occur currently in the area, which encompasses localities with current or recent (last 20-30 years) records where suitable habitat at appropriate altitudes remains.

Possibly Extant: There is no record of the species in the area, but the species may possibly occur, based on the distribution of potentially suitable habitat at appropriate altitudes, although the area is beyond where the species is Extant (i.e. beyond the limits of known or likely records), and the degree of probability of the species occurring is lower (e.g. because the area is beyond a geographic barrier, or because the area represents a considerable extension beyond areas of known or probable occurrence). Identifying Possibly Extant areas is useful to flag up areas where the taxon should be searched for.

Possibly Extinct: The species was formerly known or thought very likely to occur in the area (post 1500 AD), but it is most likely now extirpated from the area because habitat loss and/or other threats are thought likely to have extirpated the species, and there have been no confirmed recent records despite searches.

Extinct: The species was formerly known or thought very likely to occur in the area (post 1500 AD), but it has been confirmed that the species no longer occurs because exhaustive searches have failed to produce recent records, and the intensity and timing of threats could plausibly have extirpated the taxon.

Presence Uncertain: A record exists of the species' presence in the area, but this record requires verification or is rendered questionable owing to uncertainty over the identity or authenticity of the record, or the accuracy of the location.

Origin

Native: The species is/was native to the area.

Reintroduced: The species is/was reintroduced within its known historical range through either direct or indirect human activity.

Introduced: The species is/was introduced outside of its known historical distribution range through either direct or indirect human activity. Does not include species subject to assisted colonisation. Includes species intentionally moved outside of its native range to perform a specific ecological function.

Vagrant: The species is/was recorded once or sporadically, but it is known not to be native to the area.

Origin Uncertain: The species’ provenance in an area is not known (it may be native, reintroduced or introduced).

Assisted Colonisation: Species subject to intentional movement and release outside its native range to reduce the extinction risk of the taxon.

Seasonality

Resident: The species is/was known or thought very likely to be resident throughout the year.

Breeding: The species is/was known or thought very likely to occur regularly during the breeding season and to breed and be capable of breeding.

Non-breeding: The species is/was known or thought very likely to occur regularly during the non-breeding season. In the Eurasian and North American contexts, this encompasses ‘winter’.

Passage: The species is/was known or thought very likely to occur regularly during a relatively short period(s) of the year on migration between breeding and non-breeding ranges.

Seasonal Occurrence Uncertain: The species is/was present, but it is not known if it is present during part or all of the year

The species present at a site that meet the criteria needed for the site to qualify as an Important Bird and Biodiversity Area (IBA) or a Key Biodiversity Area (KBA). Note that many other species may occur at the site alongside the qualifying species

Any bird species whose global range is 50,000 km2 or less is classified as restricted range. Range restriction can also apply separately to each life cycle stage in the case of migratory species.

A global partnership that strives to understand and promote bird species that are “lost”—i.e. that have not been confirmed alive by photographic, audio or genetic information for over 10 years in the wild, and which have no ex-situ population under human care. A variety of reasons may lead to a species becoming lost. For example, they may be hard to identify or detect, live in remote locations that are rarely visited by researchers and birdwatchers, be on the verge of extinction, or have already gone extinct.

A population is considered severely fragmented when most of its individuals are found in small and relatively isolated subpopulations. These small subpopulations may go extinct, with a reduced probability of recolonization. In the IUCN Red List Criteria, the term 'severely fragmented' refers to fragmentation of the population, which often results from, but is different to, habitat fragmentation.

Species are classified into three groups based on their ecological dependence on wetland and marine habitats:

Seabirds: Species for which a large proportion of the total population relies on the marine environment for at least part of the year, based on expert knowledge of each species’ ecology (Croxall et al. 2012). This includes:

• Penguins (Sphenisciformes)
• Albatrosses, petrels and their allies (Procellariiformes)
• Tropicbirds (Phaethontiformes)
• Loons (Gaviiformes)
• Gannets and boobies (Sulidae)
• Frigatebirds (Fregatidae)
• Skuas (Stercorariidae)
• Auks (Alcidae)
• Most gulls, terns and skimmers (Laridae)
• Most cormorants (Phalacrocoracidae)
• Most phalaropes (Scolopacidae)
• Some pelicans (Pelecanidae)
• Some grebes (Podicipedidae)
• Some ducks (Anatidae)

Waterbirds: Species that are ecologically dependent on wetlands, as defined by the Ramsar Convention on Wetlands. This includes:

• Loons (Gaviiformes)
• Grebes (Podicipediformes)
• Flamingos (Phoenicopteriformes)
• Storks (Ciconiiformes)
• Screamers, swans, geese and ducks (Anseriformes)
• Hoatzin (Opisthocomiformes)
• Finfoots (Heliornithidae)
• Cranes (Gruidae)
• Pelicans (Pelecanidae)
• Cormorants (Phalacrocoracidae)
• Darters (Anhingidae)
• Avocets and stilts (Recurvirostridae)
• Jacanas (Jacanidae)
• Oystercatchers (Haematopodidae)
• Plovers (Charadriidae, Dromadidae, Pluvianidae, Pluvianellidae)
• Seedsnipes (Thinocoridae)
• Ibisbill (Ibidorhynchidae)
• Plains-wanderer (Pedionomidae)
• Limpkin (Aramidae)
• Sunbittern (Eurypygidae)
• Hamerkop (Scopidae)
• Shoebill (Balaenicipitidae)
• Most herons (Ardeidae)
• Most ibises and spoonbills (Threskiornithidae)
• Most rails, gallinules and coots (Rallidae)
• Most coursers and pratincoles (Glareolidae)
• Most gulls, terns and skimmers (Laridae)
• Most sandpipers, snipes and phalaropes (Scolopacidae)
• Most painted-snipes (Rostratulidae)
• Most thick-knees (Burhinidae)

Landbirds: Species that are not ecologically dependent on wetlands or marine environments. Includes all other orders and families not listed above, as well as some non-wetland dependent members of some of the waterbird families.

Please note that these three broad groups are not entirely mutually exclusive, and some species have two codings to reflect their ecological association with e.g. wetland and marine habitats. A wider range of species may use wetland and marine habitats opportunistically, but do not meet the criteria for ecological dependence outlined above. To retrieve data for all species recorded in a particular habitat, please use the “Habitat” filter in the Data Explorer.

Geographically or otherwise distinct groups in a population between which there is little demographic or genetic exchange (typically one successful migrant individual or gamete per year or less). Species may be at greater risk when the population is subdivided into many small spatial units or when most individuals are concentrated into one such unit.

Direct threats to species and sites are coded using the IUCN Threats Classification Scheme, which uses a hierarchical structure with three levels.

For each threat impacting a species, the following details are recorded:

Timing: Ongoing; past (unlikely to return); past (likely to return); future; or unknown

Scope: the proportion of the population (within the site, when applied for threats to sites) affected by the threat, coded as whole (>90%); majority (50-90%); minority (<50%); or unknown

Severity: the rate of population decline caused by the threat within its scope, coded as causing very rapid declines; rapid declines; slow, significant declines; causing/could cause fluctuations; negligible declines; no decline; or unknown

Ongoing threats are assigned an impact score based on their scope and severity (using the methods outlined in Garnett et al. 2018).

In addition to the direct threats identified as actual or potential drivers of population change, accompanying non-exclusive stresses are coded with each threat to highlight how each threat impacts the population. These follow the IUCN Stresses Classification Scheme.

Consumptive use of birds—for example as food, medicine or pets—is common in many parts of the world. Information on use and trade is included in the IUCN Red List assessment of relevant species to highlight the species’ importance to human health and livelihoods, and to inform the extinction risk assessment. It is important to note that utilisation of a species does not always pose a threat—where exploitation is unsustainable and is driving population decline, it will be listed in the threats section of the Red List assessment.

End uses for species that are harvested from the wild are coded using the IUCN General Use and Trade Classification Scheme.