Native Hawaiian forest birds are being threatened by global warming, which is making it easier for mosquitoes carrying avian malaria to reach higher elevations, government researchers say.

Scientists are particularly worried about birds that are more specialized in their feeding such as some native honeycreepers, including the iiwi.

Many scientists now believe a major factor influencing the decline in bird populations is the presence of mosquitoes carrying bird-prone diseases such as avian malaria.

Most native forest birds in Hawaii have retreated to upper elevations, where colder temperatures keep mosquitoes at bay.

Yet a few species have returned to lower elevations, leading scientists to speculate that they have developed a resistance to the disease.

The most diverse communities and abundant populations of native forest birds live in forests above 5,525 feet, in elevations where transmission of avian disease is limited by the temperatures.

But as upper elevations become warmer, the mosquitos are expected to advance upslope, leaving native birds with a smaller safe habitat.

On the islands of Kauai and Hawaii, there is evidence that transmission of avian malaria might be increasing in these high forests.

"It does not look good," said Dennis LaPointe, a research ecologist with the U.S. Geological Survey. "If temperatures begin to creep up, the research suggests there’s going to be a narrower band of forest that the birds can exist in without avian diseases."

LaPointe, who works at the Pacific Island Ecosystems Research Center, said the forest birds at risk include some honeycreepers, including the iiwi, a bright reddish-orange bird with a long, curved, salmon-colored bill and orange legs.

But the Hawaii amakihi, a native honeycreeper, is expanding in some low-elevation habitats on Hawaii island despite high rates of avian malaria.

Researchers said information points to the amakihi evolving a tolerance to the disease.

"We think this has emerged just recently," said research microbiologist Carter Atkinson.

He said researchers are capturing four to five times as many amakihi at lower elevations than higher elevations — the opposite situation for most native birds. For instance, the amakihi has been in Manoa since the 1990s, he said.

Atkinson said the amakihi has the avian malaria parasite in its blood but doesn’t get as sick as many other native birds.

"As a result, their mortality (death rate) is lower," he said.

Atkinson said there are indications other forest birds, including the apapane, have developed a tolerance, but researchers don’t have the evidence to prove it right now.