What’s Killing Hawaiʻi’s Forest Birds?
In the last 200 years, 17 out of 41 species of Hawaiian honeycreeper have gone extinct. Another 14 are endangered.
In the early 1900’s, a silent killer began stalking the natural and cultural heritage of Hawaiʻi. From the mountains to the shoreline (mauka to makai), the forests of Hawaiʻi were falling silent. Something was killing Hawaiʻi’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.
Naturalists began to record the disappearances of many endemic Hawaiian species in the late 1800ʻs. Habitat loss and introduced predators such as 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 Hawaiʻi’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.
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.
Avian malaria itself probably came to Hawaiʻi 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 Hawaiʻi’s 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 mosquitos populations, the birds have disappeared.
Mosquitoes thrive at lower and warmer elevations where they infect birds with avian malaria and pox. Higher and cooler elevation forests, where mosquitoes do not thrive, have become the only refuges for Hawaiʻi’s forest birds, but even those areas are now under threat. As temperatures rise, mosquitoes have begun to spread upwards into the mountains constricting habitat.
“For decades, we had reached this equilibrium,” said Josh Fisher, U.S. Fish and Wildlife Service invasive species biologist. “The birds had moved into the upper elevation forests where they had this area of refuge from mosquitoes and avian malaria, but now that is being threatened.”
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 of following. On the island of Kauaʻi, every species of native forest bird is in decline. On the island of Hawaiʻi, ninety percent of the ‘i’iwi’ population is confined to a narrow band of forest on the windward slopes, 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 Fisher. “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 Kauaʻi. There really isn’t anywhere else four these birds to go. They can’t go down and they can’t really go up much higher.”
Researchers have watched as avian malaria reduced bird rangers and pushed population declines across all of the low elevation forest of Hawaiʻi. Unfortunately, while scientist know what the problem is, there are no tools immediately available to combating avian malaria. It is a tool that still in the process of being invented.
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 Kīlauea 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 Hawaiʻi 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 Hawaiʻi’s forest birds. Read the other stories in the series at:
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.
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The U.S. Fish and Wildlife Service works with others to conserve, protect, and enhance fish, wildlife, plants, and their habitats for the continuing benefit of the American people. For more information, connect with us through any of these social media channels at https://www.facebook.com/PacificIslandsFWS, www.flickr.com/photos/usfwspacific/, or www.twitter.com/USFWSPacific.