Sado Wildlife in Focus | Why Nearly 100 Ibises Vanished in a Year
Sado Wildlife in Focus | Why Nearly 100 Ibises Vanished in a Year
野生動物を守る取り組みは、日本だけでなく世界各地で続けられています。新潟県の佐渡島でも、トキと人が共に生きる環境をつくろうとする努力が続いてきました。けれど自然の世界では、思い通りにいかないことも少なくありません。人の活動と自然環境のバランスを考えるとき、私たちはどんな視点を持つ必要があるのでしょうか。
1.Article
Directions: Read the following article aloud.
At the February meeting of the Toki (Japanese crested ibis) Reintroduction Review Committee, an announcement left me stunned. I say that as someone who has spent years on Sado Island photographing and observing these birds.
Since their first release into the wild in 2008, their population had been steadily increasing. So when the committee reported that their numbers had suddenly dropped by roughly 100 in a single year, I was desperate to understand what had happened.
The Japanese crested ibis is a nationally designated Special Natural Monument, but it had gone extinct in the wild. Plans for its return to Japanese skies are overseen by the environment ministry's Toki Reintroduction Review Committee, formed by a panel of experts.
At a meeting in Tokyo on February 9, the committee decided that ibises will be released in Izumo City, Shimane Prefecture, around June 2027. Following Noto in Ishikawa Prefecture, this will be the second release site on Japan's main island.
Successfully releasing the birds requires steady efforts, including cooperation from farmers and understanding from local residents. For example, reducing pesticide use in rice paddies so that small animals, which serve as food for the ibises, can survive and help create a suitable habitat.
After the meeting, Izumo Mayor Toshiyuki Iizuka held an enthusiastic press conference, and the decision received widespread coverage in the Japanese media.
Meanwhile, the committee reported that, as of the end of December 2025, an estimated 473 ibises were living in the wild on Sado Island. The estimate for the same period in the previous year had been 576, meaning roughly 100 birds had disappeared in a single year.
For me, having moved to Sado to observe and photograph the ibises, this was far bigger news.
According to the environment ministry, four factors are thought to explain the decline.
The first is a change in the method used to estimate the population.
Previously, the estimate was calculated through statistical analysis based on the number of individuals whose survival had been confirmed within the past six months through leg bands and other markers.
Because the calculation is done every December, birds released in autumn (after July) were automatically counted as alive, even if they had not been confirmed, since they fell within the six-month window.
As the gap between estimated numbers and reality grew larger, this "six-month rule" was abolished. Instead, methods such as simultaneous counts at communal roosts are now used to produce more accurate estimates.
The second factor is the reduction in the number of birds released. In the past, 30–40 birds were released twice a year, but last year only nine were released.
In addition, during the 2025 breeding season, there was persistent bad weather such as heavy rain, and chicks were also preyed upon by predators. As a result, the number of chicks that successfully fledged fell from 129 the previous year to 76, a drop of more than 50.
Furthermore, 17 years have passed since the first release, and the number of birds reaching the end of their natural lifespan is increasing.
Another factor came to mind. In previous years, ibis flocks typically formed large groups of 20–30 birds, but last autumn they seemed smaller — around 10 birds — which I had found puzzling.
A weekly magazine had also reported that the number of ibises on Sado had grown so much that the population was becoming overcrowded.
When the number of birds increases, food must be shared among more individuals, and birds may start interfering with each other's breeding. This can lead to a natural decline in reproductive success, a phenomenon known as the "density effect."
Environmental changes are also a factor. The area of rice paddies on Sado was about 6,000 hectares when the first birds were released, but it has since decreased to about 5,000 hectares, reducing feeding grounds as well.
Professor Emeritus Hisashi Nagata of Niigata University, who has studied the ecology of the ibis for many years, says, "Last year, the density effect reduced both breeding success and survival rates. Revising the estimation method has brought the numbers closer to reality."
He adds that under Sado's current environmental conditions, around 500 birds may be the optimal population size.
Nature exists in a delicate balance. Until now, release programs have been designed mainly with the goal of increasing the population. But it may be time to shift policy toward release plans that match the environment's capacity.
In May, the first release on Honshu, Japan's main island, is planned in Noto, Ishikawa Prefecture. From now on, the stage for ibis releases will gradually move from Sado to the Japanese mainland.
Since their first release into the wild in 2008, their population had been steadily increasing. So when the committee reported that their numbers had suddenly dropped by roughly 100 in a single year, I was desperate to understand what had happened.
The Japanese crested ibis is a nationally designated Special Natural Monument, but it had gone extinct in the wild. Plans for its return to Japanese skies are overseen by the environment ministry's Toki Reintroduction Review Committee, formed by a panel of experts.
At a meeting in Tokyo on February 9, the committee decided that ibises will be released in Izumo City, Shimane Prefecture, around June 2027. Following Noto in Ishikawa Prefecture, this will be the second release site on Japan's main island.
Successfully releasing the birds requires steady efforts, including cooperation from farmers and understanding from local residents. For example, reducing pesticide use in rice paddies so that small animals, which serve as food for the ibises, can survive and help create a suitable habitat.
After the meeting, Izumo Mayor Toshiyuki Iizuka held an enthusiastic press conference, and the decision received widespread coverage in the Japanese media.
Meanwhile, the committee reported that, as of the end of December 2025, an estimated 473 ibises were living in the wild on Sado Island. The estimate for the same period in the previous year had been 576, meaning roughly 100 birds had disappeared in a single year.
For me, having moved to Sado to observe and photograph the ibises, this was far bigger news.
According to the environment ministry, four factors are thought to explain the decline.
The first is a change in the method used to estimate the population.
Previously, the estimate was calculated through statistical analysis based on the number of individuals whose survival had been confirmed within the past six months through leg bands and other markers.
Because the calculation is done every December, birds released in autumn (after July) were automatically counted as alive, even if they had not been confirmed, since they fell within the six-month window.
As the gap between estimated numbers and reality grew larger, this "six-month rule" was abolished. Instead, methods such as simultaneous counts at communal roosts are now used to produce more accurate estimates.
The second factor is the reduction in the number of birds released. In the past, 30–40 birds were released twice a year, but last year only nine were released.
In addition, during the 2025 breeding season, there was persistent bad weather such as heavy rain, and chicks were also preyed upon by predators. As a result, the number of chicks that successfully fledged fell from 129 the previous year to 76, a drop of more than 50.
Furthermore, 17 years have passed since the first release, and the number of birds reaching the end of their natural lifespan is increasing.
Another factor came to mind. In previous years, ibis flocks typically formed large groups of 20–30 birds, but last autumn they seemed smaller — around 10 birds — which I had found puzzling.
A weekly magazine had also reported that the number of ibises on Sado had grown so much that the population was becoming overcrowded.
When the number of birds increases, food must be shared among more individuals, and birds may start interfering with each other's breeding. This can lead to a natural decline in reproductive success, a phenomenon known as the "density effect."
Environmental changes are also a factor. The area of rice paddies on Sado was about 6,000 hectares when the first birds were released, but it has since decreased to about 5,000 hectares, reducing feeding grounds as well.
Professor Emeritus Hisashi Nagata of Niigata University, who has studied the ecology of the ibis for many years, says, "Last year, the density effect reduced both breeding success and survival rates. Revising the estimation method has brought the numbers closer to reality."
He adds that under Sado's current environmental conditions, around 500 birds may be the optimal population size.
Nature exists in a delicate balance. Until now, release programs have been designed mainly with the goal of increasing the population. But it may be time to shift policy toward release plans that match the environment's capacity.
In May, the first release on Honshu, Japan's main island, is planned in Noto, Ishikawa Prefecture. From now on, the stage for ibis releases will gradually move from Sado to the Japanese mainland.
Source：Sado Wildlife in Focus | Why Nearly 100 Ibises Vanished in a Year
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