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What’s Killing Hawaii’s Forest Birds?

In the last 200 years, 17 out of 41 species of Hawaiian honeycreeper have gone extinct. Another 14 are endangered.

Extinct Hawaiian Honeycreepers (Left to Right): Greater Koa Finch (Rhodacanthis palmeri), Kauai 'O'o (Moho braccatus), Hawaii 'Akialoa (Hemignathus obscurus), Hawaii 'O'o (Moho nobilis)

From: Birds of the Sandwich Islands, By Wildon Scott (1899) Courtesy of the Smithsonian Institute

In the early 1900’s, a silent killer began stalking the natural and cultural heritage of Hawaii. From the mountains to the shoreline (mauka to makai) the forests of Hawaii were falling silent. Something was killing Hawaii’s honeycreepers - a unique family of birds not found anywhere else on the planet.

The ancestors of the honeycreepers arrived on the newly formed Hawaiian islands millions of years ago and quickly evolved to fill the empty landscape with dozens of new and unique species, a process called adaptive radiation.

An feeds i’iwi on a lobeliad flower. Photo Credit: Jack Jeffrey

Naturalists began to record the disappearances of many endemic Hawaiian species in the late 1800’s. Habitat loss and introduced predators like cats, rats, and pigs were taking their toll on all of the native plant and animal populations.

But as the decades passed and the species continued to disappear, it seemed as if something other than habitat loss and predation was impacting Hawaii’s birds - particularly the honeycreepers. The huge flocks of ‘I’iwi that once roamed the mountainsides foraging for food had disappeared, even from seemingly pristine lowland forest.

Endangered Hawaiian Honeycreepers (Left to Right) Akiapolaau © Jack Jeffrey Photography, Akohekohe © Jack Jeffrey Photography, Amakihi © Jack Jeffrey Photography, Hawaii Akepa © Jack Jeffrey Photography

By the 1960’s biologist Richard Warner believed he had identified the culprit. The answer lay in the pattern of disappearance. The birds were disappearing from healthy lower elevation forests but were still present in the higher elevation forests where the temperatures were cooler. Whatever was killing the birds, it was only happening at lower elevations. The culprit, he theorized, was a disease found commonly in bird species across the planet - mosquito borne avian malaria.

Apapane with a mosquito. Photo © Jack Jeffrey Photography

Avian malaria itself probably came to Hawaii in the early 1900’s when domesticated birds were imported into the lowland cities. These introduced birds almost certainly carried avian malaria, but the disease never would have spread to the native bird population if it hadn’t been for another earlier invader - the mosquito Culex quinquefasciatus. The Culex quinquefasciatus species is the only known vector of avian malaria. Without Culex, the disease cannot spread.

In the vast majority of bird species, the parasites (members of the genus Plasmodium) have little to no impact on the health of the infected birds. Long exposure to the parasite has allowed most species around the world to evolve effective disease resistance.

For many of Hawaiiʻs forest birds, which evolved in the isolation of the world’s most remote archipelago without exposure to malaria or mosquitoes, the impacts have been catastrophic. Wherever the forest birds range overlapped with mosquito populations, the birds have disappeared.

Mosquitos breed and thrive at lower and warmer elevations where they infect birds with avian malaria and pox. Higher and cooler elevation forests, where mosquitoes and their diseases do not thrive, have become the only refuges for Hawaii’s forest birds, but even those areas are now under threat. As temperatures rise, mosquitoes, and the avian diseases they carry, have begun to spread upwards into the mountains further constricting habitat.

“For decades, we had reached this equilibrium. The birds had moved into the upper elevation forests where they had this area of refuge from mosquitoes and avian malaria, but now that's being threatened,” said Josh Fisher, Invasive Species Biologist with the Pacific Islands Fish and Wildlife Office .

Today, nearly every species of Hawaiian honeycreeper is facing shrinking ranges and declining populations. In the last 200 years, 17 out of 41 known species of honeycreeper have gone extinct. Another 14 are endangered. On the island of Kauai, every species of native forest bird is in decline. Ninety percent of the ‘i’iwi’ population is confined to a narrow band of forest on the windward slopes of the island of Hawaii, between 4,000 and 6,000 feet (1,300 and 1,900 meters) in elevation. “We are at this point where some of these populations are so low or so dependent on a single area, that a single catastrophic event could spell the end of a species” said Josh Fisher, Invasive Species Biologist with the Pacific Islands Fish and Wildlife Office .

“There’s an urgency now that didn’t exist before because warming temperatures are already starting to push mosquitoes into the upper elevations of places like Kauai. There really isn't anywhere else for these birds to go. They can't go down and they can't really go up much higher.”

For decades researches have watched as avian malaria reduced bird ranges and pushed population declines across all of the low elevation forests of Hawaii. “We’ve known what the problem is, but with avian malaria there isn't a tool that we can just pull out of a box to fight this. We have to develop the tool,” said Fisher.

Places like Hakalau Forest National Wildlife Refuge are tackling habitat loss by aggressively planting native species and restoring forests. James Campbell National Wildlife Refuge and Kilauea Point National Wildlife Refuge are using predator proof fences to keep out invasive predators and protect birds. But introduced diseases are more difficult to address. “Fortunately over last decade there have been significant advances in how to address mosquitoes at the landscape scale,” said Fisher.

Biologists and conservationists across Hawaii are working to find ways to save the honeycreepers: from dealing with predation by introduced species to landscape scale control of mosquitoes. Now more than ever, it is important to work with our partners to protect these species for future generations.

This story is part of a series examining the issues impacting Hawaii’s forest birds. Read the other stories in the series here.

Hawaii’s Magnificent Forest Birds

ʻŌhiʻa lehua: the Foundation of Hawaii’s Forest Ecosystem

Want to dive deeper?

Collapsing avian community on a Hawaiian island Paxton, Eben; Camp, Richard J.; Gorresen, P. Marcos; Crampton, Lisa H.; Leonard, David L.; VanderWerf, Eric

Abundance, distribution, and population trends of the iconic Hawaiian Honeycreeper, the ʻIʻiwi (Vestiaria coccinea) throughout the Hawaiian Islands Paxton, Eben H.; Gorresen, P. Marcos; Camp, Richard J.

Large-Scale Range Collapse of Hawaiian Forest Birds under Climate Change and the Need 21st Century Conservation Options. Fortini LB, Vorsino AE, Amidon FA, Paxton EH, Jacobi JD

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