A Hawaiian bird’s two-part bill is only part of its drilling toolkit

The ʻakiapōlāʻau, nicknamed the Aki, has a bill that appears to combine two different tools. In a Popular Science report by Margherita Bassi, researchers describe how the small Hawaiian bird uses its long, slender upper bill to pull food from narrow spaces, while its short, robust lower bill helps it drill into wood. The unusual pairing lets the bird fill a woodpecker-like ecological role in Hawaii, where woodpeckers do not live.

Alexander Clark, an evolutionary biologist and functional anatomist at the University of Chicago and an affiliate of the Field Museum, led the study, which Bassi reports was published in the journal Ornithology. Clark described the Aki as the only bird currently known to use its upper and lower bills for separate tasks that each depend on the respective part. That qualification matters: the claim reflects current knowledge, not proof that no other bird could have a comparable arrangement.

The study’s reported anatomical detail adds a surprise: the bill is not the whole drilling system. Researchers found that the upper bill’s inner bony structure is much shorter than its outer sheath. They also identified adaptations elsewhere in the body, including a relatively large tendon associated with a muscle that flexes the ankle.

Clark’s explanation, as reported by Bassi, is that the Aki brings its body into the strike. Woodpeckers mainly use neck muscles to draw back and lunge forward; the Aki instead lifts its body and bends its ankles to drive downward. The researchers interpret the leg and ankle anatomy as evidence that this forceful gouging motion begins below the head. The report describes a body-wide movement from feet to bill, though it does not provide the study’s methods or measurements for evaluating the mechanics in detail.

That connection between anatomy and behavior helps explain why the bird’s appearance is more than an oddity. The different bill shapes serve different functions, while the rest of the body supports the wood-gouging task. Together, they offer an example of how an animal’s anatomy can be specialized across multiple body parts for its way of finding food. Clark and his colleagues’ account makes the Aki’s distinctive adaptation easier to understand as an integrated system rather than a strange-looking bill alone.

The evolutionary explanation remains an interpretation. Study co-author Paul Banko, a former researcher at the USGS Pacific Island Ecosystems Research Center in Hawaii, said the species likely evolved amid intense competition for resources. Clark also described Hawaii’s isolation as a source of exceptional evolutionary adaptations and fragility. The report does not establish a single cause for the Aki’s traits, but presents competition as part of the context in which they may have evolved.

The species is listed as endangered. The report says scientists estimate fewer than 2,000 remain in the wild, all on one slope of one mountain on the island of Hawaii. A highly specialized bird confined to such a small area illustrates the vulnerability that can accompany island endemism: distinctive adaptations can arise in isolated ecosystems, while the species that carry them may have a narrow range.

<a href="https://www.popsci.com/environment/aki-bird-frankenstein-beak-hawaii/">Source: Popular Science report</a>