The breeding grounds of a number of native bird species, including skylarks and shrikes, have sharply decreased over the past two decades, but several species imported originally as pets are becoming established in the wild, according to an interim report on bird habitats released Tuesday.

Some 1,700 members of the Wild Bird Society of Japan conducted a nationwide study for the Environment Agency from 1997 to 1998 into the breeding grounds of 600 types of wild birds in Japan, according to the report.

The breeding grounds of seven of them, however, including the once common triller, skylark and thick-billed shrike, have declined markedly since the previous survey in 1978, the report says.

The society members found only 71 breeding grounds for skylarks, compared with 211 in the previous study. The breeding grounds vanished mainly in northern Kyushu and the Kansai region, the report says.

The number of breeding grounds of the thick-billed shrike dropped from 20 to one, found in Aomori Prefecture, while triller breeding grounds fell from 37 to eight.

Trillers and thick-billed shrikes were added to the agency's list of endangered species in 1998.

The agency said the decline in breeding grounds reflects the loss to development projects in fields and along rivers that had been nesting grounds.

Another factor is a decline in the population of certain species that winter in Southeast Asia, due to deforestation there.

Meanwhile, a breeding ground for the Peking robin, native to southern China and the Himalayan region, was found for the first time in Japan, in Ibaraki Prefecture.

The survey also found a number of places in Kyushu where the bird appears to be establishing breeding grounds.

The Hwamei, a bird originally imported from China's Yunnan Province, has settled in western Tokyo and other areas, the study found.

The agency believes the Hwamei population in the wild increased as a result of dealers releasing the bird due to poor sales, an increase in bushes suitable for breeding, and the continuation of warm winters.

Commenting on the overall results of the study, Masae Narusue, a researcher at the society, said it became clear that ecosystems are being disrupted and biodiversity is being lost.

She warned that birds such as shrikes will disappear from Japan unless urgent protective measures are taken.