Hummingbirds are the only birds that are able to fly backwards.
They can also fly side to side, up and down, and hover in place like a helicopter. Rather than simply flapping their wings up and down like regular birds, who can only fly forwards, hummingbirds rotate their wings in circles or in figure-eights. They also fly at an average speed of 25-30 mph. To fly like this, hummingbirds have to move their wings much faster than other birds. They flap their wings anywhere between 12 to 90 times per second and their heart rate can reach as high as 1,260 beats per minute. 30% of a hummingbird's weight is its muscles for flight.
Hummingbirds are constantly eating in order move this fast, and at any given time they are hours away from starving to death. They have the highest metabolism of all animals (except for insects), and in a given day they may visit 1,000 flowers for nectar.
Hummingbirds don't really sing, like most birds do. The sound you're most likely to hear is a buzzing sound which, like a bumblebee, is the sound of their wings flapping.