Air carrier Captain reported a ground conflict while holding short of the runway due to a UAS flying around the runway. The pilot stayed in position until the UAS was out of the takeoff path.

Narrative

Holding short of runway XXC for Takeoff when I spotted what appeared to a turkey vulture flying in a strange manner compared to their normal flying styles. It was coming in our direction at about 30- 50 feet high. When it got closer I noticed it was not a vulture but what looked like a remote controlled model aircraft or glider. I asked my First Officer if he saw the same thing and did confirm what I was seeing and reported it to tower controllers.It had red elliptical wings as thin as a kite's wings with a pencil thin fuselage and a white triangular rudder on top and bottom of the tail. It was the size of a turkey vulture wing span and length. I could not see any type of a motor box that a propeller(s) would come off of and/or a housing for control surfaces that would attach to a regular model aircraft.It looked more like a glider. There was enough wind and thermals for a glider to sustain flight using thermals like the vultures were doing that day to stay aloft. It circled around the beginning of XXC and flew down the runway staying just to the left of the runway; and circled around the far end maybe at taxi 1 or 2 then flew back on the same side. Tower finally picked it up when it was circling around taxi ways and followed it back to the beginning of [runway] XXC. We held in position of XXC till it was safely out of our takeoff and climb out path.Make sure airspace round airports are drone/UAS are free zones unless authorized to fly close to landing and departing aircraft.