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How do scientists know a bird is extinct?
The key question is not simply when the bird was last seen. It is how reliable the records are and how effectively people have searched since then. For an EX assessment, IUCN guidance calls for exhaustive surveys across all known or likely habitat throughout the species’ historical range. Searches must take place at appropriate times of day and seasons or years, and continue for a period appropriate to the species’ life cycle and life form. IUCN Red List Categories and Criteria
A failure to find a bird is meaningful only if the survey had a good chance of detecting it. Searchers consider how much habitat they covered and how detectable the species would be there. A conspicuous bird in accessible habitat may be easier to survey than a quiet, camouflaged bird that skulks in remote or hard-to-reach microhabitats. Body size, movement, shyness, vocalization, social behavior, seasonal patterns, and local abundance can all affect detection. IUCN assessment guidance
What evidence goes into an extinction assessment?
IUCN guidance describes two complementary ways to assess extinction probability. One examines the history of records and surveys; the other examines threats. Considering both helps prevent a conclusion based too heavily on only one kind of evidence. IUCN assessment guidance
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Records and surveys
Assessors examine the timing and reliability of records, then evaluate targeted searches conducted after the last known record. They ask whether surveys were comprehensive and appropriate to the bird’s habits, habitat, and likely times of activity. A long gap in sightings is more persuasive when capable observers have repeatedly searched the right places under suitable conditions.
Threats
Assessors also consider the severity, duration, and geographic scope of threats, and how those pressures interact with the species’ life history and susceptibility. Threat evidence can make extinction more plausible, but threat exposure alone does not establish that every individual has died.
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BirdLife International describes a 2018 assessment by Butchart and colleagues that evaluated 61 potentially or confirmed extinct bird species using the timing and reliability of records, the timing and adequacy of surveys, and the timing, extent, and intensity of threats. Its article reports an estimate that 187 bird species have gone extinct since 1500; that figure is attributed to BirdLife’s summary of Butchart et al. (2018), not to a count made for an individual assessment. BirdLife International
How long does a bird have to go unseen?
There is no universal number of years without a sighting that automatically makes a bird extinct. Survey duration must fit the species’ life cycle and life form, and the assessment also depends on search coverage, detection probability, records, and threats. A bird may remain undiscovered despite surviving, especially if it is elusive or lives in habitat that is difficult to search.
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- Dimension(in): (L) x (W) x (D)
For EX, assessors record the year the species was last seen, but that does not reveal the exact date its final individual died. IUCN supporting-information guidance explains that extinction or possible extinction is generally considered to have occurred sometime between the last-seen year and the first assessment date in one of those categories. IUCN supporting-information guidance
Extinct, Extinct in the Wild, and Possibly Extinct
| Status | What it means | How it handles uncertainty |
|---|---|---|
| Extinct (EX) | IUCN’s definition is that “there is no reasonable doubt that the last individual has died.” The conclusion relies on exhaustive searches in known or likely habitat across the historical range. | High certainty is required; a long absence of records by itself is not enough. IUCN Red List Categories and Criteria |
| 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 historical habitat failing to record it. | Survival is known, but not as a wild population within its past range. IUCN Red List Categories and Criteria |
| Critically Endangered (Possibly Extinct), CR(PE) | BirdLife uses this tag for certain Critically Endangered species when the balance of evidence suggests they are likely extinct. | The evidence indicates likely extinction, but is not sufficient for a global EX declaration. BirdLife International |
“Possibly Extinct” is therefore not the same as “confirmed extinct.” It communicates a serious likelihood while preserving the distinction between strong evidence and the certainty required for EX.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the distinction matters
An extinction assessment is a judgment about current risk, not a forecast of the year a species will disappear. The IUCN notes that species listed as Extinct have later been rediscovered alive—the so-called “Lazarus” cases. It also warns of the “Romeo error”: a mistaken extinction declaration may lead to protective measures and conservation funding being withdrawn. IUCN Red List FAQs
For that reason, rarity, habitat loss, or an extended absence of sightings cannot by themselves establish EX. The assessment must bring together credible records, searches designed around the species’ detectability, and an evaluation of threats, while distinguishing likely extinction from the high-certainty conclusion that no individual remains.
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