An endangered honeycreeper found on the Island of Hawaiʻi has evolved in a unique way to drill into branches for food, according to scientists who have completed the most in-depth look ever into its anatomy.
The researchers discovered that the ‘akiapōlāʻau (pronounced ah-key-ah-pole-ah-ow) uses not just its distinctive beak, but its entire body to drill into branches for food.
The ‘akiapōlāʻau certainly has an eye-catching beak. “The ‘akiapōlāʻau is unique from every other bird that has ever evolved, and not just in Hawaiʻi – it has features unseen in any birds,” says Alexander Clark, a PhD candidate at the University of Chicago and lead author of the study. “Its lower bill and its upper bill are distinctly very different from each other.”
Its bills essentially operate as two distinct tools, giving the bird its nickname of the ‘Swiss Army knife of the Hawaiian islands’.
Its upper beak is long, flexible and slender, which allows the bird to probe grubs and caterpillars out of nooks and holes in branches. Its bottom beak is short and thick and is used to gauge holes into wood. Together, these allow the ‘akiapōlāʻau to function similarly to woodpeckers.
Scientists have long theorised that intense competition for resources had likely driven the evolution of this bird, but they didn’t know exactly how.
“We knew what they’re doing, but we didn’t quite know anatomically how their body is facilitating those movements. How are they actually doing it? And now, we have a pretty good idea,” explains Clark.
Using historical specimens, the researchers have provided a detailed description of the bird’s anatomy; published in the journal Ornithology. They discovered that the bony projection in the ‘akiapōlāʻau’s upper bill is much shorter than its external structure. The bird also has proportionally shorter wings than its mainland counterparts, as well as a proportionally shorter femur (thigh bone) that the researchers note mirrors the evolution of woodpeckers.
But it was the ‘akiapōlāʻau’s foot and ankle structure that most interested the researchers. It has a proportionally large tendon for a muscle that flexes its ankle, which gives it the ability to quickly and powerfully bring its body downwards – using the full momentum of its body to gouge into wood.
“It’s like if you’re swinging an axe to split a log, you’re putting your whole body in there,” explains Clark. “You’re elevating yourself; you’re almost on your tippy-toes and then coming down. So is the ‘akiapōlāʻau. They rear up and use their whole body but that whole body downward force starts with dorsiflexion of the ankles. It starts with the legs – the anchor system of what they’re actually hammering.”
Hawaii has high rates of endemism, and the ‘akiapōlāʻau is found nowhere else on Earth. It’s listed as Endangered on the IUCN Red List, with researchers estimating that wild ‘akiapōlāʻau populations are below 2,000 individuals.
