This endangered Hawaiian bird with a "Swiss army beak" uses its whole body to drill for food
There are no woodpeckers on the Hawaiian islands, but a little yellow bird called the ‘akiapōlāʻau (Ah-key-ah-pole-ah-ow) has evolved a crazy beak in order to drill into branches to find food. A new study in the journal Ornithology provides the most in-depth look ever into this bird's unique anatomy, including its "Swiss army beak" that looks like two different birds' beaks, Frankensteined together. Researchers learned that this critically endangered bird's beak isn't the only thing that helps it get its meals—its whole body, from its ankles on up, are made for drilling. Their findings also highlight the unique native ecology of the Hawaiian islands and the urgent need to fight the extinction of its endemic species.
“The Aki is unique from every other bird that has ever evolved, and not just in Hawaii—it has features unseen in any birds,” says Alexander Clark, a PhD candidate at the University of Chicago, affiliate of the Field Museum, and lead author of the study. “Its lower bill and its upper bill are distinctly very different from each other.”
Its upper and lower beak are so different from one another that they essentially operate at two separate tools, hence the nickname the “Swiss Army Knife of the Hawaiian islands.” The upper beak is long, slender and flexible, perfect for probing grubs and caterpillars out of small nooks and holes in branches. The bottom mandible is short, stout, and used for gouging holes into wood. Together these parts allow the Aki to occupy a niche similar to that of woodpeckers, which never made it to the Hawaiian islands.
“For a bird like the ‘akiapōlāʻau to evolve, there had to have been intense competition in the ecosystem,” says Paul Banko, co-author of the study and former researcher at the USGS Pacific Island Ecosystems Research Center in Hawaii. “When the island of Oahu emerged from under the water, there was a relatively large explosion of animal species. This was undoubtedly due to the different species competing for valuable resources, like caterpillars, to feed their young.”
But even with this understanding of the why the Aki had developed such extreme features, scientists have been puzzled as to the how. Many of the Aki’s specific anatomical features that allow them to hunt for insects in this way without causing injury to themselves was not obvious to the scientists who study them.
“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,” says Clark.
New findings include a detailed description of the bony rostrum in the upper bill, which they found is substantially shorter than the external structure. The Aki also has proportionally shorter wings than its mainland counterparts and a proportionally shorter femur that mirrors the evolution of unrelated woodpeckers.
Most substantial was the discovery related to the Aki’s foot and ankle structure. The study found that the Aki has a proportionally large tendon for a muscle that flexes the ankle, giving it the ability to quickly and powerfully bring its body downward. Woodpeckers, when they drill into surfaces, primarily use their neck muscles. But this finding tells us that Aki, in comparison, uses the full momentum of its body to gouge into wood, starting at its feet.
“Basically, it’s like if you're swinging an ax to split a log, you're putting your whole body in there. You're elevating yourself, you're almost on your tippy toes and then coming down. So is the Aki. 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.”
“It's really cool to study this species while it's still alive,” says Clark, who spent 15 months in Hawaii studying endemic bird species. Today he often studies and describes fossil bird specimens at the Field Museum. “It's one thing to infer the function of strange adaptations based on fossil specimens, but researching this phenomenon in a living species means that it's something you can go and witness. We can observe the behavior.”
“The better academic grip we have on how anatomical form informs function, biology is all the better for it. It's really important to not only understand how these things are living, but what's allowing them to move and behave the way that they do. These small, almost imperceptible internal changes make external changes in behavior and ecology–that's how the evolutionary process happens.”
“Hawaii has incredibly high rates of endemism, meaning many of the species that live on the islands are not found anywhere else in the world,” says Clark. “It is also the most isolated island chain on the planet right now. With these island ecosystems, you have such amazing examples of evolution and unique adaptations, but with this isolation, you also get incredible fragility.”
Experts estimate that the wild ‘akiapōlāʻau population is less than 2,000 individuals; all residing on the northeastern slope of just one mountain on the island of Hawaii. This study comes at a critical time for the species.
The terrestrial native species on the Hawaiian archipelago have been in decline for centuries, but the rates of extinction have increased in recent decades.
“Hawaii is very unique in that you are confronted with actual extinction, it’s something that I still struggle with,” says Clark. “This is an objective, unequivocal, complete loss of these birds' songs and of their behaviors. This is the reality that we're living in, this aspect of complete loss in Hawaii that I haven't witnessed anywhere else.”
The data in this study was obtained through the measurement of historical museum specimens across three institutions. Special thanks to collections at the UC Berkeley museum of zoology, Washington University Burke Museum, and the Field Museum.