Almost every forest bird in Hawaiʻi is spreading avian malaria

The findings, published February 10 in Nature Communications, detected avian malaria at 63 of 64 locations tested statewide. These sites included forests with very different mixes of bird species. The illness is caused by the generalist parasite Plasmodium relictum and has played a central role in the steep declines and extinctions of native Hawaiian honeycreepers.

"Avian malaria has taken a devastating toll on Hawaiʻi's native forest birds, and this study shows why the disease has been so difficult to contain," said Christa M. Seidl, mosquito research and control coordinator for the Maui Forest Bird Recovery Project, who conducted this research as part of her PhD at the University of California, Santa Cruz. "When so many bird species can quietly sustain transmission, it narrows the options for protecting native birds and makes mosquito control not just helpful, but essential."

How Avian Malaria Harms Native Birds

Avian malaria attacks red blood cells, which can lead to anemia, organ failure, lower survival rates and, in some species, death. The consequences have been severe for Hawaiʻi's iconic birds. Studies show that ʻiʻiwi, also known as the scarlet honeycreeper, face a mortality rate of about 90 percent if infected. The ʻakikiki, a honeycreeper native to Kauaʻi, is now considered extinct in the wild largely because of the disease.

Many infectious diseases rely on just a handful of species to keep spreading. This research shows that avian malaria operates differently in Hawaiʻi. Most forest birds, whether native or introduced, are at least moderately capable of infecting southern house mosquitoes, which serve as the disease's primary vector. Even birds carrying very small amounts of the parasite were able to infect mosquitoes. That means a wide range of bird communities can maintain ongoing transmission.

"We often understandably think first of the birds when we think of avian malaria, but the parasite needs mosquitoes to reproduce and our work highlights just how good it has gotten at infecting them through many different birds," Seidl said.

Chronic Infections Drive Ongoing Transmission

Researchers examined blood samples from more than 4,000 birds across Kauaʻi, Oʻahu, Maui and Hawaiʻi Island. They paired those field data with laboratory experiments to measure how readily mosquitoes became infected after feeding on birds. The results showed that native and introduced birds often had similar levels of infectiousness, meaning both groups contribute to spreading the parasite.

The study also found that birds can carry chronic infections for months or even years. During this extended period, when birds may appear only mildly infected, they remain capable of passing the parasite to mosquitoes. The researchers estimate that this long-lasting, low to moderate infectious stage accounts for most transmission statewide.

Climate Change Shrinks Safe Havens

The parasite's ability to infect many bird species likely explains why avian malaria is so widespread across Hawaiʻi. The findings suggest that few mosquito-infested habitats remain free of transmission risk. Conditions are becoming even more challenging as warming temperatures allow mosquitoes and avian malaria to expand into higher elevation areas that once served as refuges for vulnerable native birds.

Seidl and the Maui Forest Bird Recovery Project are members of Birds, Not Mosquitoes, a collaboration of academic, state, federal, non-profit and industry partners working to advance mosquito control in support of Hawaiian bird conservation.

The Maui Forest Bird Recovery Project operates under the Pacific Cooperative Studies Unit in the College of Natural Sciences. All birds in the study were captured and handled by trained ornithologists under state/federal permits.