What started as a straightforward cat removal operation on the Ogasawara Islands has turned into one of the most surprising wildlife stories of the decade, with a critically endangered pigeon multiplying almost tenfold in just a few years.
On tiny islands, a quiet wildlife experiment
Far south of Tokyo, the Ogasawara archipelago looks like a subtropical postcard. Steep cliffs, dense forests, coral-fringed bays. But for decades, this fragile ecosystem hid a brutal reality: feral cats hunting native wildlife around the clock.
Among their favourite prey was the red-headed wood pigeon of Ogasawara, a stocky, forest-dwelling bird found nowhere else on Earth. By the early 2000s, this pigeon was on the brink. Birdwatchers could count the remaining animals almost one by one.
Local authorities and conservationists decided on a drastic move. Over several years, they tracked, trapped and removed 131 stray and feral cats from key islands in the chain. The goal was modest: stop the bleeding and prevent immediate extinction of the pigeon and other native species.
Instead of a slow, fragile recovery, the pigeon population responded like a compressed spring suddenly released.
New research led by Kyoto University, published in the journal Communications Biology, shows how dramatic that release has been.
A near-extinct pigeon multiplies its numbers
Before the large-scale removal of cats, the red-headed wood pigeon existed in what scientists call a “remnant population”. Surveys counted only 111 adult birds and just 9 juveniles. One storm season, one disease outbreak or one bad breeding year could have finished them off.
Three years after the last cats were taken off the islands, conservation teams went back to count again. The figures shocked them:
- Adult pigeons: from 111 to 966 individuals
- Juvenile pigeons: from 9 to 189 individuals
- Total increase: close to a tenfold rise in the breeding population
For a bird that lays relatively few eggs and lives in a small, isolated habitat, this pace of growth is extremely rare. Researchers usually expect a slow, fragile climb in such situations, not a near-exponential curve.
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The red-headed wood pigeon now ranks among the fastest documented recoveries for a bird that was flirting with extinction.
The genetic puzzle: why didn’t inbreeding wipe them out?
The surprise is not only numerical. From a genetic standpoint, the Ogasawara pigeon should have been in serious trouble.
When a population shrinks to a few dozen breeding pairs, inbreeding tends to rise. Individuals share more and more of the same genes, including harmful ones. Over time, fertility drops, deformities appear and resilience to disease weakens. Many island birds and mammals have disappeared for exactly this reason.
Kyoto University’s team analysed DNA samples from the pigeons before and after the cat removal. They looked for signs of low genetic diversity, inbreeding and accumulation of harmful mutations.
They expected a picture of a genetically fragile species, kept alive only through intensive management. What they found was almost the opposite.
Hidden strength in a tiny gene pool
The pigeons did show reduced diversity, as any small isolated species does. Yet the level of inbreeding depression — the negative impact of close relatedness — remained surprisingly low.
Several factors may explain this resilience:
- Historic purging of harmful mutations: Over generations, individuals carrying the worst genetic defects may have failed to reproduce, slowly cleaning the gene pool.
- Strong natural selection: Intense predation by cats might have left only the fittest birds, concentrating robust traits.
- Behavioural mating patterns: The pigeons may instinctively avoid pairing with close relatives, even in a small population.
Against expectations, the species did not collapse under its own genetic weight once the predators were gone.
This finding challenges a long-held idea in conservation biology: that tiny populations are almost always doomed by their genes, even when threats are removed.
Why removing predators worked so quickly
Cats were introduced to the Ogasawara Islands by humans, mainly to deal with rats and as companions on ships. Once on land, they turned into efficient, nonstop hunters. Ground-nesting birds, naïve to mammalian predators, stood little chance.
With the cats gone, three things shifted almost overnight:
| Factor | Before cat removal | After cat removal |
|---|---|---|
| Nest survival | Many nests raided, few chicks fledged | Far more chicks survive to leave the nest |
| Adult mortality | High, especially at night | Reduced, more birds reach breeding age |
| Breeding effort | Disturbed, with failed attempts | More successful clutches each season |
In simple terms: every egg suddenly had a much higher chance of turning into a breeding adult. Because birds can attempt multiple clutches over their lifespan, those percentages add up quickly.
The study suggests that once a key pressure such as predation is removed, some species may rebound far faster than management plans typically assume, provided there are still enough individuals left to breed.
What this means for other endangered species
The Ogasawara pigeon story offers more than a feel-good moment. It raises questions that reach well beyond Japan’s territorial waters.
Conservation projects often face tough budget choices. Should funds go to captive breeding, genetic rescue, habitat restoration or predator control? The rapid rebound of this pigeon suggests that in certain island systems, removing invasive predators can deliver major returns within a decade.
This case hints that some species are more genetically robust than their tiny numbers imply, if key threats are addressed fast.
That does not mean every endangered bird will respond in the same way. Some have already lost too much genetic diversity. Others suffer from multiple overlapping threats: habitat loss, pollution, disease and climate stress, alongside predation.
Yet the Ogasawara data give conservationists something rare: hard numbers showing that decisive action can pull a species back from the edge, not just slow its decline.
Unpacking a few key concepts
What scientists mean by “genetic diversity”
Genetic diversity refers to the range of different versions of genes within a population. A wider mix gives a species more ways to cope with change — new diseases, shifting climate, altered habitats.
Low diversity does not automatically doom a species, but it narrows its options. In the case of the Ogasawara pigeon, the gene pool is still limited, yet apparently not so damaged that the birds cannot bounce back once external pressures ease.
“Inbreeding depression” in practice
Inbreeding depression happens when related individuals mate and rare harmful mutations become more common in their offspring. Symptoms can include:
- Lower fertility
- Higher chick mortality
- Weaker immune systems
- Physical deformities
The new research suggests that the red-headed wood pigeon has, for now, avoided the worst of these effects. Future monitoring will show whether that resilience holds as the population keeps expanding.
Lessons from a cat-free island future
The Ogasawara project will likely feed into global debates on feral cat control, a sensitive issue in many countries. Cats play a cherished role as pets, yet they are also recognised as a leading cause of bird and small-mammal declines, especially on islands.
Scientists working on the project stress that the result is not an argument for hostility toward domestic cats. Instead, they see it as a case study for targeted management in places where native wildlife has no natural defences against introduced predators.
Future scenarios now being modelled look at how the pigeon population might behave over the next 20 to 50 years. Will it stabilise at a sustainable level, overshoot available food resources or face new threats such as heatwaves and stronger typhoons linked to climate change?
Whatever happens next on those remote Japanese islands, one message stands out: once the 131 cats were gone, an entire ecosystem responded with a speed and intensity that even seasoned researchers did not anticipate.