The Bee Hummingbird (Mellisuga helenae) weighs approximately 1.8 grams, less than a U.S. penny. It is the smallest bird in the world, measuring just 2.2 inches long.
Despite the popular myth, it's impossible to seriously injure somebody by dropping penny from an airplane or a tall building. The penny is not very aerodynamic and it doesn't have enough mass to cause damage (only about 3 grams). The worst it could do is cause a sting on the victim's head.