Humans have previously seen the crows making the tools in artificial situations, in which scientists baited feeding sites and provided the raw tools; but researchers say the New Caledonian crows have never been filmed doing this in a completely natural setting.
"New Caledonian crows are renowned for their unusually sophisticated tool behavior," the study authors write. "Despite decades of fieldwork, however, very little is known about how they make and use their foraging tools in the wild, which is largely owing to the difficulties in observing these shy forest birds."
Study author Jolyon Troscianko of the University of Exeter in England described the tropical birds as "notoriously difficult to observe" because of the terrain of their habitat and their sensitivity to disturbance, he said in a press release.
"By documenting their fascinating behavior with this new camera technology, we obtained valuable insights into the importance of tools in their daily search for food," he added.
For this study, the researchers attached small cameras to the tail feathers of 19 wild crows. Just four of the crows used tools on camera, but "they did so extensively" for foraging, including on the forest floor, the authors write.
The cameras also captured two instances of birds fashioning "one of their most complex" tools, the hooked stick tools, the authors write.
"In one scene, a crow drops its tool, and then recovers it from the ground shortly afterward, suggesting they value their tools and don't simply discard them after a single use," Troscianko said.
Study co-author Christian Rutz of the University of St. Andrews in Scotland said the team also saw the birds stashing their beloved tools.
"Crows really hate losing their tools, and will use all sorts of tricks to keep them safe," Rutz said in a statement. "We even observed them storing tools temporarily in tree holes, the same way a human would put a treasured pen into a pen holder."