Hummingbirds are the wold's smallest bird. The Bee hummingbird of Cuba is only 2 inches long. This is just a little bigger than penny. Hummingbirds live only in the western hemisphere. Most migrate north in the early spring and return south in early fall. Their life span is between nine and twelve years. The colorful birds are very curious. They are also aggressive toward each other. Perhaps their best known trait is their ability to fly backwards. They are the only bird in the world that can do this.