The American military’s extensive use of drones against the Islamic State and other terrorist groups has resulted in a shortage of Air Force pilots and other personnel to operate the aircraft, leading the Pentagon to rely more on private contractors for reconnaissance missions in Afghanistan and Iraq. The number and identities of contractors working on the drone flights are considered classified information, the Air Force said. But Pentagon officials said there are at least several hundred contractors, many of them former drone or fighter pilots who are making double or triple their military salaries.