There are now only 500 or so Japanese golden eagles living in the wild as environmental changes deprive the birds of their habitat, according to a study by a research group.
The Society for Research of Golden Eagle Japan warns that the subspecies is facing extinction after a gradual decline in numbers since 1986.
"The biggest issue is the reduced breeding success rate due to a lack of prey," said Toshiki Ozawa, the group's head. Thick vegetation is believed to make it hard for the birds to spot potential prey such as hares.
Research by the Tokyo-based society shows that the number of breeding pairs has fallen by one-third over the past 33 years, and their rate of reproductive success has also decreased.
Japanese golden eagles live in mountain areas from Hokkaido to Kyushu, with each breeding pair staking out territory and remaining there for life. This characteristic has allowed the society to keep tabs on the survival of each pair and their success or failure in breeding.
When the society started monitoring numbers in 1981, it estimated the numbers at 340 pairs.
But as of two years ago, 99 of the monitored pairs had died or otherwise disappeared from their territories. Of the 241 remaining pairs, just one was found in Kyushu, and none in Shikoku.
The eagles' breeding success rate was stable at around 50 percent through the 1980s, but in 1991 it began to average between 20 and 30 percent. The breeding success rate in 2013 was 20.2 percent.
Poor forest management in the areas where the eagles live has left vegetation too dense for them to spot prey from above.
A public-private sector project aiming to halt the subspecies' decline is set to begin experimental tree thinning in parts of a government-owned forest this fall.
The Nature Conservation Society of Japan, a nongovernmental organization, has collaborated with the Forestry Agency for the project to thin out parts of the Akaya Forest, which stretches across 10,000 hectares in Niigata and Gunma prefectures.
Japanese golden eagles have a wingspan of up to 2 meters and typically hatch two chicks, of which only one usually makes it out of the nest. As well as being vulnerable to habitat changes due to human development, the birds are prone to abandoning their nests when disturbed.