Nina Foster / 10 Jun 2025
The ʻŌʻū used to be one of the most common and widespread Hawaiian birds, found in lowland and montane forests on the islands of Kaua‘i, O‘ahu, Moloka‘i, Maui, Lānaʻi, and Hawai‘i. During the twentieth century, however, a lethal combination of mosquito-borne avian diseases, habitat loss and degradation, and predation by non-native cats and rats caused the ʻŌʻū population to plummet. The IUCN Red List declared the species threatened in 1988 and critically endangered in 1994.
Jack Jeffrey, a wildlife biologist and longtime Hawai‘i resident, has had multiple encounters with the ʻŌʻū. The olive-green bird was often difficult to spot flitting through treetops, but its repetitious, canary-like series of whistles and trills rang clear through the forest. In 1989, Jeffrey became one of the last people to see the plump, thick-billed honeycreeper. It had been years since the bird was sighted, and over a decade since the last known documentation of the species in 1977. He found the bird on Kaua‘i with Andrew Engilis, Jr. and Thane Kastle Pratt, who published their findings in Hawai‘i Wildlife Newsletter later that year.
At first, Jeffrey was optimistic. “When you see something that hasn’t been seen for a long time, to me, it’s a signal of hope,” he reflects. He didn’t realize that their report would mark the last confirmed sighting of the species. A few years later, Hurricane Iniki hit. “That devastated the island. There was very little chance of survival,” he adds.
Recent searches for the ʻŌʻū, alongside extensive surveys for other species in its range, have failed to locate the lost bird. As of 2024, the ʻŌʻū is lost forever, declared extinct in an IUCN Red List update. Four other lost species, including three honeycreepers endemic to Hawai‘i, have met the same fate: Kauaʻi nukupuʻu, Maui nukupuʻu, Maui ‘Ākepa, and White-chested White-eye.
This year’s update to the lost birds list celebrated the rediscovery of eight species, but the extinctions are a poignant reminder that many species remain out of reach. Confirming the existence or absence of lost birds can be a challenging, at times depressing, process — but the work provides crucial information for conservation efforts and policies that prevent further losses.
Confirming Extinction
According to the IUCN Red List, a taxon becomes extinct “when there is no reasonable doubt that the last individual has died.” There is a common misconception that this occurs after a 50-year period of no confirmed sightings. Yet several birds have been rediscovered after disappearing from the scientific record for decades, even centuries. Take, for instance, the Black-browed Babbler, which was first described and collected by scientists in the 1840s but not observed again until 170 years later, when locals in Borneo helped track down the bird.
Moving a species to the extinct category is not a decision that Red List Authorities take lightly. “Intuitively, the worst thing you can possibly do is declare something extinct when it isn’t, because when you declare something extinct, you are signaling that people shouldn’t bother looking for it,” says Alex Berryman, Red List Officer at BirdLife International, the Red List Authority for birds. On the other hand, not declaring a species extinct when it is may result in the unnecessary use of conservation resources and the underestimation of extinction rates.
It’s impossible to say with certainty whether the last individual of a species has died, but researchers have developed two models to determine the likelihood of extinction based on a number of parameters. These models allow Red List Authorities to more consistently and confidently classify species as critically endangered or extinct.
The first model focuses on the threats responsible for a taxon’s decline, taking into account the geographic extent of conservation concerns as well as their capacity to drive both local and global extinctions. The second considers all known surveys and records of a taxon. A survey is a dedicated or passive effort to find a species, while a record refers to any evidence that the species still existed in a given year — think labeled museum specimens, photographs, or local sightings. For each survey, Red List Authorities evaluate the study’s scope as well as the probability that the species would have been recorded and reliably identified. They also assess each record’s reliability using criteria such as the type and quality of evidence, the circumstances of the record, and the skill and experience of the recorder.
Each model returns a probability of extinction. If the probability of either model is less than 0.5, the species is considered critically endangered but still extant. If the output of both models is between 0.5 and 0.9, the species qualifies as critically endangered (possibly extinct). Extinction can be declared if both models yield a probability above 0.9. The ʻŌʻū had an extinction probability of 0.941 based on threats and 0.984 based on records and surveys.
The “possibly extinct” tag was proposed by Butchart et al. in 2006 to create a subset of critically endangered species that are likely extinct but should not be listed as such due to insufficient evidence. “It communicates the fact that people should still look for this species,” explains Berryman. “We’re missing a bit of information, and there is a possibility that it’s out there.”
These models and probability thresholds can be conservative, keeping some species in the critically endangered category that are actually extinct. But the process helps avoid a phenomenon known as the “Romeo Error.” This term refers to a situation in which a species is declared extinct only to be rediscovered years later, at which point it is too late to save the remaining individuals, and the species truly disappears. “The error is that in those few years you were calling it extinct, when it wasn’t, you could have implemented conservation action and possibly saved it,” adds Berryman.
The Search Continues
Searching for birds that are likely extinct may seem like a waste of resources, but exhaustive surveys are essential for Red List Authorities to make informed decisions about the conservation status of a species. They cannot declare a species extinct if people haven’t looked for it. But there can be risks to this conservative approach if extinction announcements lose their impact due to a lengthy time lag between the last observation of a species and the declaration of its extinction. “It’s like reporting a murder that happened decades ago,” says Dan Lebbin, Vice President of Threatened Species at American Bird Conservancy (ABC). “This increases the importance of searches happening rapidly for species as a form of evidence of absence, so these declarations can happen with less lag time.”
The best-case outcome of a lost bird search is locating the target species, as in the case of every lost bird rediscovery. This presents an opportunity to take action to benefit the species’ conservation. But even if the target species is not found, searches often yield valuable ecological findings and community connections.
Expeditions to look for the Sinú Parakeet exemplify some of the benefits of these “unsuccessful” searches. The parakeet, which was last documented in 1949, has been lost to science for more than seven decades. Sociedad Ornitológica de Córdoba (SOC), with the support of ABC and the Search for Lost Birds, has led multiple expeditions to find the lost species in Alto Sinú in the Department of Córdoba, Colombia. These efforts have yet to discover the parakeet, but they have generated important knowledge about the region’s biodiversity, established valuable partnerships with local communities, and motivated conservation action.
In 2021, an expedition in search of the Sinú Parakeet marked the first comprehensive ornithological survey for the Alto Sinú, a region previously inaccessible to researchers due to Colombia’s decades-long civil conflict. The team documented 238 bird species, including 30 that had never been documented in the department, as well as other rare plants and wildlife. Among the recorded birds was the rare Crested Eagle, a large forest raptor with an extensive range but sparse distribution. While speaking with local communities about the parakeet, the team learned of a Crested Eagle nest in the forest — the first ever recorded in Colombia. The following year, the eagle pair returned to build a second nest and successfully raise a chick.
The Crested Eagle sighting inspired the establishment of La Cristalina Reserve, which protects 656 acres of tropical forest where the eagle’s nest was found and provides important habitat for more than 300 bird species. “All of it started with the Sinú Parakeet,” reflects Eliana Fierro-Calderón, ABC’s International Conservation Project Officer. “The species has become a symbol of everything that has happened: being able to be in the area, work with the communities, create a reserve, protect birds, and learn all these new things about the ecosystem.”
Last September, a sighting of a bird that looked like the Sinú Parakeet prompted two new expeditions in pursuit of the species, one that month and another in February 2025. These surveys recorded 195 bird species in the Alto Sinú, including close relatives of the Sinú Parakeet. On the second expedition, the team also learned of a photograph from the late twentieth century that, if found, would be the only definite photographic evidence of the parakeet and radically move up the bird’s last date of documentation.
“The results so far have provided us with valuable information that gives us a clearer route to continue our search,” writes Eduar Paez, a biologist and ornithologist who led the most recent expeditions with SOC. “Although the road remains challenging, there is much to explore and we are more committed than ever to rediscovering the Sinú Parakeet.”
Looking Forward
In an ideal world, the story of every lost bird would end in rediscovery. Unfortunately, habitat loss and degradation, climate change, invasive species, and other threats are putting the survival of hundreds of bird species at risk.
“Extinction is not just dinosaurs and dodos, things of the past. Extinction is happening today,” emphasizes Chris Farmer, Hawai‘i Program Director at ABC. In Hawai‘i, the bird extinction capital of the world, ABC and local partners are racing to save species in decline. Their work spans multiple islands, targeting the threats that led to the loss of the ʻŌʻū and other Hawaiian birds.
Although extinction declarations can be discouraging, these announcements have the potential to raise public awareness, garner support and funding for conservation efforts, and spark proactive measures that safeguard the future of species still in existence. After all, the extinction of North American wildlife — including the Carolina Parakeet, Great Auk, and Passenger Pigeon — led to the enactment of the Endangered Species Act and other groundbreaking conservation legislation.
Extinctions are a call to action, underscoring the urgency of bird conservation. The extinction of the Slender-billed Curlew, for instance, should be a wake-up call. The species has yet to be evaluated by the IUCN Red List, but a recent study concluded that the shorebird has become the first known bird species from continental Europe to go extinct. The Slender-billed Curlew may be lost forever, but there are other extant curlew species and shorebirds that we still have time to save. “We need to protect those shorebirds so we don’t lose more,” says Lebbin.
Above all, it’s important to celebrate the wins as they come. 2024 brought more lost bird rediscoveries than extinctions, and the Search for Lost Birds will continue to work toward finding and conserving species in need of protection. “The reality that some Lost Birds turn out to be extinct highlights just how important and exciting it is when a species is rediscovered,” says John Mittermeier, Director of the Search for Lost Birds at ABC. “I am hopeful that we have many more rediscoveries waiting to happen among the 121 species that are currently lost.”
Nina Foster is a science communication specialist with interests in ornithology, forest ecology, and sustainable agriculture. She holds a BA in English literature with a minor in integrative biology from Harvard University and is currently the Post-baccalaureate Fellow in Plant Humanities at Dumbarton Oaks.