Which is the only bird able to fly backwards?

Hummingbirds fly backwards because their wings work differently from every other bird's. A typical bird generates lift on the downstroke and mostly recovers on the upstroke. A hummingbird rotates its wing at the shoulder joint, so the wing turns nearly upside down and produces lift on the return stroke as well, tracing a figure of eight through the air. That gives it a hovering flight closer to an insect's than a bird's, and once you can hover and control the tilt of that stroke plane, moving backwards is simply a matter of angling it. The reason is food. Hummingbirds feed from flowers that offer no perch, so they must hold position at a bloom and then reverse out of it. The energy cost is enormous, with a heart rate and metabolism among the highest of any vertebrate, and many species drop into a torpid state overnight to avoid starving before morning.

Hummingbirds live only in the Americas, which is why they are absent from European and Asian gardens.