Drones operated by researchers may have unintended consequences for wildlife, warns the lead author of a new study showing the buzzing of unmanned aerial vehicles overhead can leave black bears stressed, with racing hearts.

Researchers flew drones about 20 metres above black bears that were wearing GPS collars and cardiac monitors to measure what effects the unfamiliar noise had on the bears.

Lead author Mark Ditmer said they thought the bears might flee, but they hardly moved at all.

Instead, their heart rates spiked, showing a major stress response.

"For them to mostly stay in one spot, and have this racing heart rate, was a little bit of a surprise for us," said Ditmer, a postdoctoral researcher in conservation biology at the University of Minnesota who led the study published in Current Biology.

Drone Mark Ditmer

Mark Ditmer watches the successful landing of a drone after a research mission involving a flight over a bear foraging in a cornfield in northwestern Minnesota. (Jessie Tanner)

Stress response 'pretty severe'

The team gathered data on the bears' movement while the drones flew, with the collars sending a new location every two minutes as the bears ambled through corn fields and aspen forests in northwestern Minnesota.

But they had to wait until the bears were in hibernation before downloading the heart rate data.

The researchers had expected some physiological reaction to the unfamiliar buzzing overhead, but not such a strong response, said Ditmer.

"I couldn't believe it," said Ditmer.

"It became strikingly obvious that we were seeing a pretty acute stress response that was pretty severe, at least in some cases."

In the most extreme case — a mother bear with two cubs — the bear's heart rate spiked to 400 per cent of her resting rate, jumping from 41 beats per minute before the drone flight to 162 beats per minute when the drone circled overhead.

That kind of stress response, which likely also included a surge of adrenalin and other changes, helps a wild animal in a real emergency, but chronically stressed individuals are more susceptible to disease and other problems, said Ditmer.

'Cautionary tale'

Ditmer and colleagues wanted to look at the bears' reaction, because drones are increasingly used in research and conservation — not to mention by hobbyists — with little known about their effects on wildlife.

This study was limited to 18 flights over the four adult bears in a zone where federal rules allowed drone flights. Other types of wildlife would respond differently to the unfamiliar sound of a drone, said Ditmer.

Still, he hopes it's a "cautionary tale" as drone use increases.

"Just because we're not noticing an animal changing behaviour, that doesn't mean there's not some sort of negative response happening."