The ʻakiapōlāʻau has no need for a toolbox. The endangered Hawaiian bird taps the wood of trees, hacks into it with the lower half of its unusual beak, then switches to its long, flexible upper bill to probe the hole for insect larvae to eat.
New research provides the most comprehensive look yet at this Hawaiian honeycreeper's unique anatomy, showing that when the bird strikes the wood, even its legs get involved in the action.
The ʻakiapōlāʻau (pronounced ah-key-ah-POLE-ah-ow) is found only on the island of Hawaii and feeds mainly on insects, including larvae hidden in branches. There are no native woodpeckers in Hawaii, but the ʻakiapōlāʻau has evolved its own way of getting at prey inside wood, filling a similar feeding niche despite the two birds not being closely related.
Scientists had long observed how the ʻakiapōlāʻau feeds, but knew much less about the anatomy that makes this behavior possible. To investigate, the researchers examined three ʻakiapōlāʻau skeletons from the Museum of Vertebrate Zoology at the University of California, Berkeley, and compared them with skeletons of related continental finches from other museum collections.
"The anatomy of an animal is like the book, whereas the in-life behavior is like the movie," said Alexander Clark, a University of Chicago doctoral candidate and scientific affiliate with the Field Museum in Chicago who was the lead author of the study published in the journal Ornithology.
In that "movie," the first step is finding where the prey may be hiding. The bird first assesses a branch and sometimes taps it gently with its upper bill to feel for hollow spaces, Reuters quoted Clark as saying.
Once it selects a spot, the bird switches tools. It opens its mouth widely and begins to vigorously gouge the woody substrate with the lower bill only, while the upper bill is held as far away as possible. After creating an opening, it inserts the slender upper bill and searches from side to side for its prey.
"With its mouth fully agape, it almost looks like the little bird is yelling at the wood it's gouging," Clark said.
The researchers found that the upper bill is unusual beneath the surface as well. Its underlying bony structure is about 38% shorter than its outer covering. The section without bone can flex as the bird moves its head from side to side while probing.
"When I've witnessed this behavior, I could've sworn it was going to snap its bill – and yet, it doesn't," Clark said.
Perhaps the biggest surprise came from the legs. The bird has a proportionally short femur, or the first bone of the leg that articulates with the hip.
"If you're constantly clinging to and needing to maintain awkward positions on perches, it helps to lower your center of gravity, particularly by reducing the leg length," Clark said.
Researchers also found a well-developed attachment for a muscle that flexes the ankle.
"This muscle, when contracted, flexes the ankle, bringing the lower part of the leg towards the body. It's essentially the same movement as if you were to stretch out your leg, and raise your foot towards your shin. You're working the muscle I'm talking about," Clark said.
When the ʻakiapōlāʻau gouges wood, that movement helps lift the body and then drive it forcefully downwards. That differs from woodpeckers, which primarily initiate their drilling movement using neck muscles to cock back and then lunge forward.
"In ʻakiapōlāʻau, it's a whole-body affair," Clark said. "Like effectively swinging an axe, you have to put your whole body into it."
Repeatedly striking wood may also carry a risk of injury, and scientists do not yet fully understand how the ʻakiapōlāʻau's skull copes with the forces involved. Woodpeckers have reinforced bone structures between the eyes that help cope with stresses from drilling. The ʻakiapōlāʻau has something similar, Clark said, but nothing to the same degree.
Mastering the technique also takes time. Young ʻakiapōlāʻau remain with adults for an unusually long period while learning to forage, Clark said.
"It takes practice to use that bill," Clark said.
With Hawaiian honeycreepers facing the risk of extinction, Clark added, "there will never again be a time in Earth's history when their songs will make up part of the dawn chorus."
