Hawaiian crows, a species extinct in the wild, have demonstrated a remarkable skill that's exceptionally rare in the animal kingdom: the ability to use tools.
The discovery, described in Nature, means there are now two species of crow that are known to use tools — and there could be more.
The other tool-wielding species, the New Caledonian crow found in the South Pacific, is famous for turning sticks into sharp pokers to probe for larvae hidden in trees.
"Until our discovery, the New Caledonian crow was a fascinating oddity in the avian world. Everyone was amazed by their tool behavior," said Christian Rutz, a behavioral ecologist at the University of St. Andrews in Scotland who has studied crows for more than 10 years.
He wondered whether the South Pacific birds were truly unique.
But with more than 40 species of crows and ravens in the world, it was possible that there were other winged tool-users out there.
So Rutz began searching. New Caledonian crows have unusually straight bills and forward-facing eyes that provide binocular vision, two features that help them handle stick tools. In what he described as "a proper eureka moment," he realized that other birds with similar features may also be able to use tools.
Rutz zeroed in on the Hawaiian crow, known on the island as the 'alala, which "show striking similarities to New Caledonian crows," he said. He did a Google image search and found compelling resemblances.
At that point in early 2013, there were only 109 'alala living in two captive breeding facilities managed by the San Diego Zoo Institute for Conservation Research. He gave them a call and booked a flight to Hawaii.
"I said, 'This may sound crazy, but I think you have a hundred or so birds that are natural tool users.' They said, 'Yeah, we have seen some using sticks,'" Rutz said. "They didn't think that was scientifically significant."
Since the entire species is housed in captivity, Rutz and colleagues were able to test almost every living Hawaiian crow. (Five birds were excluded due to health concerns.)
The crows were tasked with retrieving a piece of meat from holes drilled into a log. Of the 104 birds tested, 78% of them — including 93% of adults and 47% of the youngsters — spontaneously picked up a stick to reach the hidden food. In most cases, the birds successfully completed their task within one minute.
The scientists observed the Hawaiian crows selecting the appropriate tool, replacing unsuitable ones and gathering up extra plant material to make new tools — similar to behaviors seen in New Caledonian crows.
To see if the tool-using behavior was not the result of mimicry by the captive birds, the researchers raised seven 'alala chicks in isolation, where they had no opportunity to learn the behavior from other crows or humans.
Eventually, each young 'alala picked up a stick to search for food.
Younger crows were not as dextrous as adults, suggesting the birds require about two years to hone their skills, Rutz said.
In the early 2000s, Hawaiian crows became extinct in the wild, but not before the remaining birds were captured and placed into a captive breeding program. There are currently 131 'alala housed at the centers on the islands of Hawaii and Maui. Later this year, some of the crows will be released to restore a wild population on the Big Island.
When that happens, Rutz and his expert team of corvid trackers will eagerly observe the birds to see whether they behave the same way they did in captivity.
"We're all hoping they use tools," Rutz said. "If they don't, we won't be hugely surprised."
There are many good reasons why the birds may not pick up sticks in the wild again, Rutz said.
It's possible the habitat conditions on Hawaii have changed so that this kind of foraging behavior is no longer profitable. The last wild 'alala may have abandoned tool use altogether as the environment forced the crows to find other ways of foraging.
The study authors could find no documented cases of Hawaiian crows using tools in the wild, except for one passing mention of a bird carrying a twig in its bill even though the nest-building season was over. Scientists may never know whether the former wild population used tools under natural conditions, the authors acknowledge.
Despite the discovery, tool use among animals remains very scarce. Less than 1% of the world's animal genera (the category just above species) use tools.
Among birds, the Hawaiian crow joins the New Caledonian crow and Galapagos woodpecker finches known to use stick tools. The Egyptian vulture somewhat more clumsily uses rocks to break open ostrich eggs. A few nonhuman primates use tools as well, including chimpanzees and capuchin monkeys.
As more species join the elite ranks of animal tool-users, scientists gain more traction in tackling questions about the evolutionary origins of tool use. Why do some crows use tools and others do not? Why did humans become so extraordinarily — and uniquely — handy?
For now, Rutz and his colleagues hope to chip away at questions about how tool use in the Hawaiian and New Caledonian crows evolved.
The last common relative of the two species lived about 11 million years ago, which means the behavior probably developed independently. But both species live on tropical islands, which could offer scientists a clue.
On both islands, there's no competition from woodpeckers for the bugs and prey that live inside dead wood and trees. Also, there are no predators.
Using tools demands much of the animal's attention, something it would not be able to do if it were constantly on the lookout for hawks or tree snakes.
"The plot is thickening that something is going on on remote tropical islands," Rutz said.
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