Bears show signs of stress when drones are flown near them, according to a new study.

Researchers at the University of Minnesota put health tracking collars on six bears, and measured data during 17 drone flights.

The heart rate of all the bears increased when the drones were flown within 20 metres of their location.

The study concluded more research was needed to see whether bears would get accustomed to drone flights over time.

Despite the rise in heart rate, the bears did not usually display a behavioural response to the drone.

"Bears in this population live in a highly human-altered landscape", the researchers wrote, "And therefore may exhibit lower stress responses."

Unmanned aerial vehicles are useful tools for wildlife researchers because they can easily visit remote locations and observe animals from a distance.

In the case of a goose-bothering drone built in Canada, they have also been used to deliberately scare off unwanted animals.

Steve Wambolt built the GooseBuster, a drone with a loudspeaker playing recordings of predatory birds.

The noise scares away geese which can be a nuisance.

Mr Wambolt told Modern Farmer tests had worked "remarkably well".

However the team behind the study on bears suggested drone-induced stress could make animals "more vulnerable to sources of mortality", such as fleeing into another animal's territory, or running into traffic.

It advised wildlife researchers to take caution when using drones "especially with regard to endangered species".

The study was published in scientific journal Current Biology.