Which is the only bird that can fly backwards?

A hummingbird can hover on the spot, fly backwards, and even briefly upside down, because its wing does not work like other birds'. The shoulder joint rotates almost freely, so the wing generates lift on the upstroke as well as the down, tracing a figure of eight rather than flapping. That is closer to how an insect flies than a sparrow.

It is expensive. A hummingbird's heart can exceed 1,200 beats a minute and its wings beat up to eighty times a second, and it lives so close to its energy limit that it must feed every few minutes through the day. At night, or when food is short, it drops into torpor - body temperature falling towards the air temperature and heart rate falling to a fraction of normal, effectively hibernating overnight to avoid starving before morning.

For all that, they migrate. The ruby-throated hummingbird crosses the Gulf of Mexico in a single flight of around 800 kilometres, non-stop over open water, on a body weighing about three grams.