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Scientists do not declare a bird extinct simply because it has not been seen for years. They assess how reliable the records are, how thoroughly and effectively people have searched its range, and whether the threats it faced could have eliminated it. Under the IUCN standard, a species is Extinct (EX) only when there is no reasonable doubt that its last individual has died.
Why a long gap in sightings is not enough
A bird may go unrecorded because it is rare, difficult to detect, or lives in habitat that has not been searched adequately. BirdLife International cautions that “a lack of recent records may be due to insufficient survey effort rather than extinction of the species.” The key question is therefore not only when the bird was last seen, but also what searches took place afterward and how likely they were to find it if it survived.
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There is no universal number of years without a sighting that automatically makes a bird extinct. The appropriate search period depends on the species’ life cycle and life form, and the conclusion depends on the full evidence rather than elapsed time alone.
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Coverage across the historical range
For an EX assessment, IUCN guidance makes exhaustive searches central: investigators should search all known or likely habitat across the species’ historical range. A survey of only one accessible site cannot establish that a bird is gone from a wider range if other suitable areas remain unsearched.
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Timing and duration suited to the species
Surveys need to take place at appropriate times of day and seasons or years, and continue for a period appropriate to the bird’s life cycle and life form. A search outside a bird’s active or detectable period can produce a false absence, even where people have visited its habitat.
How detectable the bird is
Scientists consider the chance that a surviving bird would be detected in the areas surveyed. Detectability can depend on the bird’s size, behavior and movement, shyness or skulking, seasonal patterns, vocalization, sociality, camouflage, local abundance, and the accessibility and searchability of its habitat and microhabitat. A quiet, cryptic bird in remote habitat requires a different survey design from a conspicuous bird in accessible habitat. A failure to find a bird is persuasive only when the searches had a strong chance of finding it.
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How records, searches and threats are weighed together
IUCN guidance describes two complementary ways to assess the probability of extinction. They examine different evidence, so one can point to a different conclusion from the other; considering both helps avoid relying too heavily on a single line of evidence.
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Records and Surveys Model
This model considers the timeline and reliability of records alongside the timing, coverage and adequacy of targeted searches after the last known record. Repeated, credible records followed by well-designed searches that fail to find the bird are more informative than an old or uncertain last record followed by limited survey effort.
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- Dimension(in): (L) x (W) x (D)
Threats Model
This model examines the severity, duration and geographic scope of threats, and how they interact with the species’ life-history traits and susceptibility. Threats can make extinction more plausible, but threat evidence is assessed alongside the records and searches rather than serving as proof by itself.
As an example of this combined approach, BirdLife International reports that Butchart et al. (2018) evaluated 61 potentially or confirmed extinct bird species using the timing and reliability of records, survey timing and adequacy, and the timing, extent and intensity of threats.
What EX, EW and CR(PE) mean
| Category or tag | What it indicates | What it says about surviving birds |
|---|---|---|
| Extinct (EX) | IUCN’s definition is that “there is no reasonable doubt that the last individual has died.” The assessment relies on exhaustive searches in known or likely habitat across the historical range, carried out at appropriate times and for an appropriate period. | No individuals are understood to survive. |
| Extinct in the Wild (EW) | The species survives only in cultivation, captivity, or as a naturalized population outside its past range. An EW presumption also depends on exhaustive searches in historic habitat failing to record it. | Survivors may exist, but not in the wild within the species’ past range. |
| Critically Endangered (Possibly Extinct), CR(PE) | BirdLife describes Possibly Extinct as a tag for certain Critically Endangered species when the balance of evidence suggests likely extinction, but is not conclusive enough for a global EX declaration. | The tag communicates a serious likelihood of extinction without treating uncertainty as proof that the last individual has died. |
“Possibly extinct” and “confirmed extinct” are not interchangeable. CR(PE) lets an assessment convey that extinction appears likely while the evidence still falls short of the certainty required for EX.
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Why uncertainty matters after a status decision
The IUCN Red List assesses current extinction risk; it does not forecast the date on which a species will disappear. For EX, EW and possibly extinct assessments, IUCN supporting-information guidance records the year last seen. Extinction or possible extinction is generally considered to have occurred sometime between that year and the first assessment date in one of those categories. That interval reflects the limits of the evidence better than a precise, unknowable date of death.
Some species listed as Extinct have later been found alive—so-called “Lazarus” cases. The opposite error also has consequences: a mistaken extinction declaration can lead to protective measures and conservation funding being withdrawn. The high certainty required for EX helps guard against that risk.
How many bird species have gone extinct?
BirdLife International’s 2003 article, summarizing Butchart et al. (2018), reports an estimate of 187 bird species extinct since 1500. The article describes the figure as based on quantitative assessments; it is an attributed estimate, not a count of every bird species for which extinction has been proven beyond doubt.
Quick Recap
Sources and further reading
- IUCN Red List Categories and Criteria
- IUCN Red List supporting information
- BirdLife International: finding fault in extinction assessments
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