Ogasawara: How 131 Cats Saved the Red-Headed Pigeon

What initially sounds like an odd footnote in animal welfare has proved to be a lesson in modern species conservation. On Japan’s remote Ogasawara Islands, the targeted removal of 131 feral cats triggered an unexpected surge in an exceptionally rare red-headed pigeon species. These birds had been close to disappearing from the world’s species list forever, before their fortunes changed at remarkable speed.

How removing cats transformed an island ecosystem

The Ogasawara Islands sit in the Pacific, several hundred kilometres south of Tokyo. Although they are part of Japan, these tropical islands feel like a small world of their own. Numerous animals found there occur nowhere else on Earth, making the region both highly valuable and exceptionally vulnerable.

Over many years, stray domestic cats had become highly effective hunters. At night, they stalked through vegetation, climbed shrubs and trees, and hunted anything that moved: lizards, insects and, above all, ground-nesting or slow-flying birds.

“For the red-headed pigeon of the Ogasawara Islands, cats had become a nightmare – and almost the final chapter in the species’ history.”

To prevent the impending loss, authorities and researchers chose a far-reaching measure: 131 free-roaming cats were to be caught and removed from the sensitive island environment. The approach involved no poison and no mass culls, but rather traps, monitoring and relocation.

Red-headed pigeon numbers rose from 111 to 966 in three years

Nobody expected the bird population to rebound so quickly. The number of adult red-headed pigeons rose from 111 to 966. Juvenile birds increased even more sharply, climbing from 9 to 189 individuals. This happened over roughly three years.

• Adult birds: increase from 111 to 966
• Juvenile birds: increase from 9 to 189
• Time period: about three years
• Main factor: removal of a key predator – the feral cat

Growth at this pace is more commonly associated with insects or small rodents than with a threatened bird confined to a limited area. Researchers therefore describe it as one of the fastest recorded recoveries ever observed in a species on the edge of extinction.

The genetic resilience of a tiny population

Biology normally follows a strict rule: when an animal species is reduced to just a few dozen or hundred individuals, inherited defects tend to accumulate. Inbreeding can weaken the immune system, reduce fertility and increase susceptibility to disease. A species often then enters a downward spiral from which recovery is extremely difficult.

That did not appear to happen to the red-headed pigeon on Ogasawara. Genetic analyses assessed at a university in Japan revealed unexpectedly robust genetic patterns. Despite the low initial number, there was no major accumulation of harmful mutations.

“The birds apparently retained enough genetic diversity to expand their population at great speed – a kind of emergency reserve in their genes.”

Researchers have drawn several possible conclusions:

• The population may once have been larger, with some of that diversity remaining in the last surviving birds.
• Pairings may have involved less closely related individuals than in many other island species.
• Strong selection pressure from predators may previously have favoured particularly resilient individuals.

Together, these factors may explain why the species did not suffer a genetic “collapse” and instead began expanding almost explosively once that pressure was removed.

Why cats are particularly dangerous on islands

Cats are known to eat birds. On isolated islands, however, this behaviour can intensify a crisis dramatically. Many island bird species have experienced few, if any, ground predators for thousands of years. They build nests on the ground, are more likely to hide than flee quickly, and often behave naïvely around unfamiliar hunters.

For a cat skilled at hunting, an island can therefore become a buffet. This pattern has been seen on islands around the world: among albatrosses in the Indian Ocean, parrot species in the Pacific and small songbirds in the Atlantic.

Factor	Consequence for island birds
No evolutionary experience of cats	Little escape behaviour or few defensive strategies
Ground-level or low nests	Easy prey for stalking predators
Small distribution ranges	Local losses have an immediate, major effect

The same pattern apparently emerged on Ogasawara. In numerical terms, 131 cats may not sound like a vast number, but on a restricted land area their presence was enough to push the red-headed pigeon close to extinction.

What Ogasawara means for global species conservation

The study sends several powerful messages for conservation worldwide. First, it demonstrates how great an impact targeted action against introduced predators can have. A relatively small intervention – trapping 131 animals – can produce a dramatic ecological response, allowing native wildlife to reclaim space.

Second, it brings the importance of genetic analysis into sharper focus. Rather than simply counting the animals that remain, researchers can look more closely at their genetic material. This can help predict whether a species is realistically able to recover after protection measures, or whether further action, such as breeding programmes, will be needed.

“Ogasawara shows that even very small remaining populations can still possess enough genetic power – if the decisive pressure is removed.”

Third, the case supports the need for early action. Had cat capture been delayed until the final birds had disappeared, even the best monitoring would probably have been unable to help. Conservation efforts in many regions face precisely this question of timing: when is the right moment to intervene decisively?

Conflicts, limits and risks of intervention

As positive as this story sounds, it also raises questions. For many people, cats are family members rather than “nuisances”. Programmes that remove stray cats from islands often encounter opposition for emotional, legal and cultural reasons.

There are also practical risks:

• Other predators could take over the role left vacant.
• Diseases could spread more easily through the increasingly dense bird population.
• Poor planning may merely displace animals instead of genuinely removing them.

On Ogasawara, the intervention appears to have been carefully prepared. The islands have relatively clear boundaries, monitoring worked and the affected bird species responded noticeably. Not every habitat on Earth can be controlled or isolated so clearly.

Lessons from the Ogasawara cats intervention

The story of the Ogasawara pigeons illustrates how interconnected an ecosystem can be. A pet originally introduced by people disrupted a whole chain of processes. When that factor is removed, nature can sometimes respond more rapidly and more powerfully than specialists expect.

Several lessons follow for conservation practice:

• Detecting introduced predators early on islands is worthwhile.
• Genetic testing should be a standard part of protection programmes.
• Measures need social support so that animal welfare and nature conservation are not set against each other.

Terms such as “genetic diversity” and “inbreeding depression” may sound dry, but they have direct consequences. They determine whether a species gains new momentum after an intervention such as cat capture, or gradually declines despite protection. In the red-headed pigeon’s case, the genetic foundation appeared stable enough to fill the gap left by the disappearance of the predators quickly.

Similar principles can be applied to other islands and even to enclosed landscapes on the mainland, such as amphibians in peatland areas or small mammals in isolated woodland. There, technical barriers, wildlife crossings or targeted fencing can have the same effect as removing 131 cats from a Pacific island: native species are given breathing space again and a genuine opportunity to save themselves.