Investigations into reported collisions by drones with civil aircraft found that they were caused instead by birds, wires and posts, or structural failures unrelated to unmanned aircraft systems , a U.S. regulator said. There has been widespread concern about the risks that the flying of drones close to aircraft can pose , and the Federal Aviation Administration said Thursday that reports of “possible” drone sightings to FAA air traffic facilities continued to increase in fiscal year 2016.