I think that 4-5 good riflemen could bring one of them down.
They would need to have considerably better rifles and sighting optics than I do.... from the article:
BAE drones are programmed to take off and land on their own, stay airborne for up to 15 hours and reach heights of 20,000ft, making them invisible from the ground.
Well, crap, that takes the thrill out of Saturday afternoon. I suppose now I am going to have to go scoop snow and break ice.