Hawaiian bird stages mysterious comeback

Nature rather than man may be saving a small, rare green bird in Hawaii from the brink of extinction. Until 10 years ago, the Hawaiian Amakihi could live only in high-elevation forests after it was driven almost completely from its primary habitat in lower regions by the spread of avian malaria. But scientists have discovered the Amakihi is thriving and breeding once again in its original habitat. What's unusual is that the resurgence is happening outside of any captive-breeding program or other human intervention.

Now scientists are trying to figure out why. Understanding how the Amakihi are surviving avian malaria may hold the key to saving other Hawaiian bird species, many of which are increasingly threatened by mosquito-borne illnesses.

The Amakihi's recovery could also have implications for North American bird populations, which struggle with another mosquito-transmitted disease, the West Nile virus.

"It is ... rare to witness the evolution of resistance to the disease and the 'comeback' of a population of birds," says biologist Patrick Hart, a member of the US Geological Survey (USGS)research team.

At least 71 species and subspecies of Hawaiian birds existed when Western explorers arrived in the Hawaiian Islands in 1778. Now, however, 76 percent of the Hawaiian bird species are either extinct or endangered, and several others are showing significant population declines.

Much of this ecological devastation is attributed to disease-carrying mosquitoes. Until 1826, Hawaii was free of mosquitoes, which were brought to the islands by Western traders and settlers. Native birds essentially disappeared from lowland forests, where the climate is favorable for mosquitoes.

Today most Hawaiian bird species survive only in high-elevation forests, where mosquitoes are rare. Amakihi are most common in native forests above an elevation of 2,000 feet, where they feed on insects and flower nectar. But at lower levels, these birds have adapted to survive, and now are thriving - despite high rates of malarial infection (60 to 90 percent).

So why is this happening?

"Perhaps the most tantalizing [hypothesis] is that the Amakihi has undergone natural selection and has evolved some level of resistance to avian malaria," says Bethany Woodworth, a wildlife biologist with the USGS.

Research by USGS scientist Carter Atkinson appears to support this hypothesis. He has tested lowland Amakihi populations and found the birds to be immune to malaria. "Clearly, the other half, who survived, have some genetically based immunity," Dr. Atkinson says.

This is very surprising because, says Dr. Woodworth, "to the best of our knowledge, there are no native Hawaiian forest bird species that are completely resistant to avian malaria."

Atkinson then tested Amakihi from high elevations that had not yet encountered malaria and found that about half later developed malaria and died.

The Amakihi research is taking on greater importance because mosquitoes in Hawaii seem to be adapting and spreading to higher elevations, threatening at least 15 native Hawaiian forest bird species, says USGS biologist Dr. Robert Fleischer.

Woodworth says that understanding more clearly how the birds are developing immunity may help scientists aid other bird species in recovering from mosquito-borne diseases.

"If we can understand what allows Hawaii Amakihi to persist, against all odds, in the face of disease, we can use this knowledge to manage species, habitats, and landscapes in order to mitigate disease," Woodworth says. One possibility, she says, is to relocate birds that are immune into susceptible populations in the hope that interbreeding will produce a bird population increasingly resistant to the disease.

"One of the things that we are working on is to try to answer the questions: Why Amakihi? What about other similar species? Are there environmental factors that have played a role?" Woodworth says.

The good news is that the ongoing recovery of the Amakihi suggests that the once ignored lowlands may now hold new significance for the Hawaiian conservation community. The Hawaiian ecosystem may still be evolving and not lost forever to native Hawaiian wildlife.