The ecosystems that contain Yellowstone and Glacier National Park have populations of grizzlies that are rebounding so much that bears are moving out from the mountains back onto the prairie, where people and agriculture now dominate the landscape.

"The drone was such an asset that I couldn't imagine doing the job without it," researcher Wesley M Sarmento said.

Deadly bear attacks can be prevented by drones, according to new research. Unmanned aerial vehicles are the "magic tools" needed to chase killer grizzlies away from people, say American scientists. Drones are far more effective than dogs at "hazing" - or distracting - the giant beasts,