Biology Reference
In-Depth Information
A kilometer along the trail in the upland forest, we passed through groves of koa.
'Ama-
kihis
and 'apapanes were about, but no 'akis. Then suddenly Liba signaled that she'd heard
the song of the 'aki. We bushwhacked for several hundred meters to where the 'akis were
calling, and there the birds sat, perched in a koa. An 'akiapōlā'au could appear as easily in an
illustrated topic of fairy tales as in a field guide. The color of the male's plumage adds a bril-
liant new interpretation to the color yellow. The 'aki uses its straight lower bill to hammer at
tree bark like a woodpecker. Then it uses its long, thin decurved upper bill like a one-armed
tweezer to pull out tasty grubs. David Wilcove describes the 'aki as a bird with “a Swiss army
knife for a bill.”
“That's not the only foraging trick up their sleeves,” Liba explained. “They can also thrust
out their lower bill so that the tips meet and use their mandibles like pliers.” She went on to
tell us about the most interesting discovery of all: these pseudowoodpeckers also behave as
sapsuckers do on the mainland. A major part of Liba's field research was devoted to trying
to understand why 'akis use “aki trees,” or sap trees. Typically the birds forage on koa trees,
but about 5 percent of the time they forage on 'ōhi'as, yet not for insects and grubs—they're
after the sap, much like North American sapsuckers. “They drill into the surface of the bark
and into the vessels in which the sap streams (the phloem). Then, while holding their slender
upper mandible out of the way and using their lower mandible as a guide, they drink the sap
with their tongue. You see them touch the tip of their bill or their tongue to the sap over and
over.”
Liba's eyes widened as she recounted her discovery of the secret life of her study bird: “If
you stick an 'aki tree with a nail, as I did many times, the sap comes flowing out, and it is
quite sweet.” But the amazing thing was that she found the opposite to be true for the 'ōhi'a
trees she tested that showed no evidence of previous hole drilling. When she poked those
with a nail, no sap emerged.
“Somehow the birds know which trees are productive. Or perhaps they make them pro-
ductive by changing the flow and pressure of the sap,” Liba commented. Many of the 'aki
trees that produce sap are covered in holes from roots to canopy. By following the birds she
had located, she found that each pair had as many as seven to ten sap trees in their particular
home range. It turns out that only about one in a thousand 'ōhi'a trees is a sap-giving tree.
