Move over New Caledonian crows, there's a new master tool-user in the corvid family.
When Hawaiian crows are feeling a bit peckish, they too use sticks and other plant matter to retrieve an out-of-reach snack, according to research published Wednesday in the journal Nature.
And now, with another expert tool-user in the crow family, scientists can begin to pose questions about the conditions that might lead to the evolution of this innovative ability.
The New Caledonian crow's masterful tool-use was caught on video last year, but "we couldn't really make sense of why they evolved the capacity that other corvid species didn't," study lead author Christian Rutz, a biologist and ecologist at the University of St. Andrews, tells The Christian Science Monitor. It "seemed like this fascinating oddity in the avian world," he says.
But all that has changed with the observation of Hawaiian crows expertly using tools for foraging, he says.
Because the Hawaiian crows are extinct in the wild, and just over 100 individuals live in captivity at the San Diego Zoo Global, the scientists were able to test almost all the living individuals, save a handful that were excluded for health reasons. And in experiments modeled off of the known behavior of the New Caledonian crows' tool-use, Dr. Rutz and his colleagues observed that 78 percent of the Hawaiian crows used tools to probe for food in crevices or other hard-to-reach places.
When they looked more closely at the data, they saw that 93 percent of the adult subjects used tools, as did 47 percent of the younger birds.
Furthermore, more than two-thirds of the birds modified or manufactured their tools.
"How incredible that they found a ubiquitous form of tool use in a species of bird that had never been documented before," says Rob Shumaker, executive vice president and zoo director of the Indianapolis Zoo, and author of the book "Animal Tool Behavior," who wasn't part of the study. "It's a terrific and an important addition to the literature," he tells the Monitor.
Rutz and his team tested the birds' abilities by building wooden structures to mimic the logs or trees Hawaiian crows might encounter in the natural world, complete with nooks and crannies too deep for the birds to reach their beaks into. Then they stuffed these crevices with small pieces of meat.
All this was set up under an opaque sheet, so the birds wouldn't get any ideas, Rutz explains. "The first thing they see once the task was set up was a foraging opportunity. And the striking result was that the vast majority of them immediately picked up sticks and started probing for the hidden food."
The scientists conducted a second experiment in which they raised seven Hawaiian crow chicks separate from the adults so they would not observe any tool use before being tested. They even went as far as to hide any human tool use from the baby birds.
"The big question was whether these birds would spontaneously, without any social input, start developing functional tool use. And they did," Rutz says. "That suggests that this species has certain genetic predispositions and presumably has a long evolutionary history of expressing these behaviors."
With two crow species now known to be expert tool-users, scientists can begin looking for patterns in the conditions conducive to this behavior.
"Both New Caledonian crows and Hawaiian crows evolved on remote tropical islands. We know that these island environments are quite unusual in terms of the ecological conditions they present," Rutz says. With less competition for embedded prey from animals like woodpeckers, the crows can exploit that ecological niche. Furthermore, there are fewer predators on these remote islands, so the crows can take the time to sit and focus on poking around a crevice without having to be so vigilant.
"It looks as if the plot is thickening," Rutz says. Although it's not a statistically significant sample size, he admits, "We now have these two species that evolved in very similar tropical island environments.… It looks like there's something special going on on these islands."
The Galapagos woodpecker finch, another avian master tool-user, fits this scenario too, he adds.
But would Hawaiian crows use tools so readily in the wild? Scientists just might get a chance to find out. After increasing the captive population, the San Diego Zoo Global and the Hawaii Endangered Bird Conservation Program are preparing to release some of the birds back into their natural habitat.
This study began on a hunch. Rutz reached out to the San Diego Zoo Global because he thought the Hawaiian crow might be a tool user that had been overlooked.
His previous research on New Caledonian crows identified that those birds have two unusual physical features that might be related to their sophisticated tool use. Unlike the typical curved crow beak, the New Caledonian crows have unusually straight beaks, which Rutz and his colleagues think helps them hold and use stick tools. The birds also have large eyes that give them a particularly large field of vision that also might help the crows see the long, thin tools perched in their beaks.
"I realized that if this is an adaptation or a pre-adaptation to tool use in this species, then maybe we can use these features for searching for undiscovered tool using species," Rutz says. So he began looking for crow species that hadn't been well studied with those two features. "I very quickly homed in on the Hawaiian crow," he says.
Tool use is most often identified by accident, when someone sees an animal picking up an object and making deliberate use of it. But now scientists can be a little more intentional in their search.
"This tells me that there's lots more to discover if we look more carefully at many, many other species," Dr. Shumaker says.
"This is really not just about crows, and this is not really just about tool use," Shumaker says. "This is about defining what is uniquely human and what is not."
People often think about tool use and tool manufacture as a uniquely human characteristic that a small handful of other animals just happen to do. But that is just not the case, Shumaker says. "And we know that we share tool use and tool manufacture with many, many, many other species."