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A new study, led by San Diego Zoo Wildlife Alliance, Smithsonian's National Zoo & Conservation Biology Institute, and additional researchers, offers a unique lens for understanding the unprecedented extinction crisis of native Hawaiian forest birds.

Just 17 out of approximately 60 species of the iconic honeycreeper remain, most of which are facing rapid decline due to avian malaria. The findings, in Current Biology, include new evidence that there is still time to save the critically endangered honeycreeper 'akeke'e—but the window is rapidly closing.

"In a race against time to save the remaining honeycreepers, necessary insights about their survival are found in their genetic makeup," said Christopher Kyriazis, Ph.D., lead author and postdoctoral researcher from San Diego Zoo Wildlife Alliance. "Our findings provide a new understanding of the last remaining individuals as recovery efforts forge on in their native forests and in human care."

Warming temperatures have enabled non-native mosquitoes, the vectors of avian malaria, to spread further up the mountains. This leaves little to no refuge for Hawaiian honeycreepers, which lack immunity to malaria. The effects of avian malaria have been especially devastating on Kaua'i island, which lacks high-elevation habitats.

Two critically endangered honeycreeper species endemic to Kaua'i, the 'akikiki and 'akeke'e, have faced population declines of more than 99% in the past two decades. Efforts to control mosquito populations by releasing reproductively incompatible male mosquitoes are currently ongoing.

There are estimated to be fewer than 100 individual 'akeke'e remaining. Meanwhile, 'akikiki are extinct in the wild, and the only remaining hope for the species is through a conservation breeding program of approximately 40 individuals at the Maui and Keauhou Bird Conservation Centers, operated by San Diego Zoo Wildlife Alliance.

Through genomic analysis of three honeycreeper species, the 'akikiki and 'akeke'e, and the extinct po'ouli, researchers provide a detailed examination of genomic diversity, inbreeding depression and extinction risk.

Key findings:

• There is still time to save the 'akeke'e from extinction: Under current conditions, 'akeke'e are likely to go extinct in the near future. However, if ongoing mosquito control efforts are successful, recovery is still possible.
• Avian malaria is driving population collapse: Recent steep declines in 'akikiki and 'akeke'e coincide with the spread of avian malaria in the late 20th century, consistent with malaria being the primary driver of population collapse.
• Hawaiian honeycreepers maintain high genetic diversity: Hawaiian honeycreepers, even the last known po'ouli individual, maintain high genetic diversity despite being critically endangered. While high genetic diversity may increase their potential to adapt to threats, it could also increase their vulnerability to inbreeding depression as population sizes decline at a rapid rate.
• Inbreeding depression is already happening in 'akikiki: Findings suggest that many of the founders of the conservation breeding program—'akikiki brought into human care in a last-ditch effort to prevent extinction—are the offspring of related parents. Inbred birds produced fewer offspring and had lower survival. This information can be used to improve mate selection and further inform breeding decisions.

The research offers a unique window into the genomic consequences of massive species decline and provides critical insight into how the damaging genetic factors, such as inbreeding depression, further exacerbate extinction risk. Findings will inform ongoing recovery efforts with the aim of averting extinction in the remaining honeycreeper species.

Insights from the genome of the extinct po'ouli underscore the importance of biobanking. In 2004, San Diego Zoo Wildlife Alliance established a cell line from the last living po'ouli. Those cells were the first viable material from an extinct species to be cryopreserved, and they remained dormant in the Frozen Zoo until recently, when researchers thawed one precious vial.

Sequencing the po'ouli genome, as seen in this study, provides evolutionary and genetic information that can benefit the conservation of other critically endangered Hawaiian forest birds.

"We will never hear the po'ouli honeycreeper's song again, but we can learn from its genetic code," said Oliver Ryder, Ph.D., director of conservation genetics at San Diego Zoo Wildlife Alliance.