The red-headed wood pigeon, found only on Japan’s remote Ogasawara Islands, has staged an incredible recovery from the early 2000s, when the bird's numbers fell to fewer than 80 individuals.
The turning point came when conservationists removed feral cats, which, since being introduced to the islands, had been preying on the red-headed wood pigeons (Columba janthina nitens). The population rebounded within just three years – an unusually fast recovery that caught the attention of researchers at Japan's Kyoto University.
Most species that are reduced to such a tiny population suffer from inbreeding where harmful genetic mutations build up. This makes recovery difficult, even with intensive conservation measures, says Daichi Tsujimoto, first author of a new study (published in Communications Biology), which analyses the recovery. But this pigeon defied the odds.
"This pigeon's exceptional rebound led us to investigate the underlying genetic reasons for its resilience, hoping to uncover what makes some endangered species more capable of recovery than others."
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Story of the red-headed wood pigeon
The red-headed wood pigeon is a medium-sized bird measuring about 40cm in length. As its name suggests, it has a reddish head contrasting with a grey-brown body. It favours mature forests and is genetically and ecologically different from the Japanese wood pigeon (Columba janthina), a subspecies widely distributed on islands near the Japanese mainland and the Korean peninsula.
Once numbering in the hundreds, the population of red-headed wood pigeons fell sharply in the late 20th century due to forest destruction and predation by introduced feral cats. Numbers dropped to fewer than 80 individuals in 2008 and the species was close to extinction.
In an attempt to reverse the downward trend, conservationists stepped in, with their focus on removing the feral cats.
"Intensive trapping of feral cats began in 2010 on Chichijima Island in the Ogasawara Islands," the study explains. "As a result, 131 feral cats were trapped between 2010 and 2013, and the cat population was reduced to less than 20 individuals.
"Over the same period, the total number of pigeons observed increased from 111 to 966 adults and from 9 to 189 juveniles."
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A resilient bird
There's no doubting the red-headed wood pigeon's remarkable recovery. But how could such a small population rebound so quickly without a build-up of harmful genetic mutations?
To find out, the team sequenced and compared the entire genomes of both wild and captive red-headed wood pigeons, along with a wild population of the more common Japanese wood pigeon. Their aim was to measure the level of inbreeding and assess the genetic load – the number of harmful mutations – in each species.
Their findings were surprising. The red-headed wood pigeon had a lower frequency of highly damaging mutations than its widespread relative. This suggests the species’ genetic resilience was rooted in its long-term survival as a small, isolated population, even before humans arrived, says team leader Yuji Isagi.
Over centuries, gradual inbreeding appears to have purged harmful mutations from the pigeon's genome – a process known as genetic purging. This rare genetic ‘housekeeping’ meant that when the population was reduced by feral cats, the birds were better equipped to survive the bottleneck and bounce back once the predator threat was removed.
"This unique evolutionary history appears to have provided these pigeons with a resilience that is not seen in other endangered populations," adds Isagi.
The study challenges the widespread belief that inbreeding always hinders recovery by exposing damaging mutations. Instead, it highlights that under certain conditions, small populations can adapt genetically over time to withstand severe declines.
However, the researchers warn that the story is not all positive. Even with its resilience, the pigeon's current numbers remain below historic levels. A smaller gene pool may limit its ability to adapt to future environmental changes, and there is still a risk of losing genetic diversity over time. Restoring the population to historic levels remains a priority.
Find out more about the study: Genetic purging in an island-endemic pigeon recovering from the brink of extinction
Top image: Chichijima Island in the Ogasawara Islands, where the study was conducted. Credit: Getty
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