Which small birds can fly backward?

Answer

Their wings rotate in a figure-eight pattern, letting them hover and fly in reverse.

Context

A hummingbird's wing joins the body only at the shoulder, a flexible joint that lets the wing rotate. At the end of each half-stroke the wings flip by more than 90 degrees, so the bird gets lift on both the downstroke and the upstroke. High-speed studies of a hovering ruby-throated hummingbird, modelled at about 42 beats per second, found the downstroke gives roughly 2.5 times the vertical force of the upstroke, so the lift is shared unequally.

The figure-eight description is a popular simplification. National Geographic's account of the motion does not use the phrase, and the modelling paper describes the wing tip following a roughly elliptical path. Reverse flight is the same rotating stroke used with the body angled differently.

Hummingbirds are the only birds described as having a true reverse gear, and the only birds able to hover in still air for 30 seconds or more; other birds can briefly back up or hover. The reverse flight lets them back away from flowers after feeding.

The cost is extreme. Wingbeat rate falls with body size, from about 80 per second in a small species to about 10 per second in the giant hummingbird, and some are reported up to about 100 per second. Their metabolic rate is the highest of any vertebrate, and heart rates can exceed 1,000 beats per minute.