As unmanned aircraft have become a popular tool for the United States military, private companies are developing new drones that can be used by law enforcement domestically. The new drones – one under development the size of a hummingbird – have police departments across the U.S. excited for their possible applications. But the idea of robotic planes spying on American citizens have civil libertarians raising concerns. The BBC’s Alistair Leithead  filed this report on the next generation of drones.