Hawaiian honeycreepers have lived in the islands' tropical forests for millennia, but the colorful finch-like birds are facing "imminent collapse" on Kauai, experts say.
Rising temperatures on the island are probably responsible, since they allow mosquito-borne diseases to spread across the island's avian community, the study authors say.
As the Earth warms, the areas suitable for mosquitoes — and the diseases they carry — are expanding. Kauai, a mountainous island covered in forest, used to be cool enough to keep diseases such as avian malaria and avian poxvirus at bay. Now the warming climate is pushing mosquito-free zones higher up the mountains, restricting prime honeycreeper habitat to small pockets.
"We suggest that a tipping point has been crossed," wrote lead author Eben Paxton, a research ecologist with the U.S. Geological Survey, and colleagues.
At the current rate of decline, the study predicts "multiple extinctions in the coming decades."
And Kauai may just be the beginning.
On Kauai, forest bird habitat lies much lower in elevation than on other islands, such as Hawaii and Maui. That means the march of mosquitoes into the upper reaches of the birds' habitat has hit Kauai first.
"Kauai represents an early warning for the forest bird communities on the Maui and Hawaii islands, as well as other species around the world that are trapped within a climatic space that is rapidly disappearing," the study authors wrote.
For now at least, the taller mountains of Hawaii and Maui still provide some refuge for certain species of honeycreeper, which generally do not migrate from island to island. According to recent climate change and disease models, however, the temperature will have increased enough to bring disease up to the highest forests by 2100, Paxton said in an interview.
"Already we are seeing declines of other species on Hawaii and Maui Islands," Paxton said. "We think that is, in part, [due to] disease slowly moving up in elevation across all the islands."
In addition to pressures from climate change and disease, the honeycreepers also face competition from non-native birds and from invasive plants that often overtake their usual food sources.
Among birds, Hawaii's forest birds are something of an evolutionary novelty.
More than 5 million years ago, a kind of Asian finch made its way across the ocean and settled on the island chain. As volcanic activity created more islands, the ancestral finches hopped from place to place, establishing new populations that eventually diverged from their founder species over millions of years.
At least 55 species of Hawaiian honeycreepers evolved, including at least one distinct species on each of the Hawaiian islands.
"A lot of people say if Darwin had landed on Hawaii, he would have gotten his theory of evolution much quicker," Paxton said.
Four of the six species currently in decline are found only on Kauai. None are found beyond the Hawaiian Islands.
In the study, researchers counted individual birds among the island's eight native forest bird species, as well as several non-native birds. They documented a 68% average population decline among six species of honeycreepers in their core habitat and a 94% average decline along the periphery of the birds' range.
"We've definitely seen a much more rapid decline in the more recent years," Paxton said. "The rates of decline are increasing."
Since 2000, these two species, along with the 'anianiau and the i'iwi, have experienced such severe range contractions that they are now limited to a remote area of the Alakai Plateau that covers no more than 25 square miles. Another endangered species, the secretive puaiohi, lives in narrow river gorges and was too poorly sampled to count.
But as the honeycreeper populations declined, a different bird, the Kauai 'elepaio, increased by 88% in its core habitat range.
The 'elepaio, a kind of monarch flycatcher, include three different species, including one found on Kauai. These three species have shown a resistance to avian malaria.
Scientists think the 'elepaio owe their immunity to the species' more recent arrival to the Hawaiian islands.
Until about the mid-1800s, mosquitoes and the diseases they carry were unknown to the Hawaiian islands. Over time, the Asian finches that first colonized the island chain might have lost their immunity to avian malaria, while the latecomers retained their natural resistance.
"All of this is new to these birds who lived for millions of years without any vector-borne diseases," Paxton said.
For the honeycreeper family, extinction isn't a new threat.
The birds have seen waves of extinctions over the centuries. When the Polynesians arrived on the Hawaiian islands roughly a thousand years ago, they introduced the rat. Then Europeans brought the black rat, a much more efficient predator. As recent as 12 years ago, the black-faced honeycreeper, the poʻo-uli of Maui, is believed to have gone extinct.
"What's going on on Kauai is a sad story," Paxton said. "There are a number of things that can be done. It may not save all the species, but I think if a big effort was made soon it could save a number of species."
The island's remote and rugged terrain makes traditional mosquito control techniques — such as pesticide spraying — impractical.
Other options include releasing sterilized male mosquitoes or introducing Wolbachia bacteria into the wild mosquito population, which can interfere with malaria infection. Each option, while promising, holds technological, social and financial hurdles, Paxton said.
"I think we can save these birds, but we need to act now," Paxton said. "And if we do, I think we can create a great framework to ensure the persistence of birds on Maui and the Big Island."
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