"With seven hours of video [from 12 individual crows] we made more new discoveries than in hundreds of hours of field observations," Rutz said.
Researchers already knew that the crows "fish" for beetle larvae in dead wood with tools made from sticks or leaves.
Video footage showed that the crows also forage extensively on the ground, using a previously unreported type of tool—stalks of grass—to turn over loose material in search of insects.
"The fact that they use tools on the ground shows that the niche they exploit with their tool use is much larger than previously thought," Rutz said.
Another discovery was that the crows did not always use whatever stick or stem was close by to serve as a foraging tool.
In one instance, Rutz said, a favored tool was used over a prolonged period of time and carried in flight from one location to another.
Each crow-mounted camera contained a radio beacon that allowed researchers to track the bird's location during and after filming and to eventually retrieve the camera.
Testing on captive crows proved that the units could be easily carried and did not affect the birds' behavior, Rutz said.
Researcher and filmmaker Greg Marshall is a pioneer in the use of animal-mounted video for research and the inventor of National Geographic's Crittercam.
(National Geographic News is part of the National Geographic Society.)
"This is the first application of one of these systems in wild birds for research purposes," Marshall said.
"These guys have pushed the frontier in terms of identifying interesting research questions that can be addressed [through animal-mounted video] over fairly short periods of time."
Because battery weight is a major constraint in ultralight video systems, Marshall noted, cameras on birds cannot currently be deployed for periods of days or weeks.
In the crow study, the cameras were set to begin filming 48 hours after they were installed. The devices captured footage for 30 to 60 minutes before losing power.
But Marshall and Rutz said that lighter and longer-lasting systems can be expected with improvements in technology.
"Very soon it will be possible to capture footage from really small species," Rutz said.
"I'm confident that in the next five years or so this will take off and become one of the standard techniques in field ornithology."
Bird expert John Rotenberry at the University of California Riverside said that "for birds large enough to support the hardware, this is a pretty promising system."
Unfortunately, he added, video tracking remains far out of reach for his own work on sparrows and other songbirds.
"[The system] will need to get down to around 300 to 600 milligrams [0.01 to 0.02 ounce] before I get to use it," Rotenberry said.
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