Hummingbirds are very small birds native to the Americas. They can hover in mid-air and fly in acrobatic ways which most other bird cannot.
Inside the bell jar in the time room of the Department of Mysteries was a tiny jewel-like egg which hatched into a hummingbird. However, since the bell jar essentially captured a time loop, the bird reverted back into a wet just-hatched chick and then was enclosed in its egg again. The pattern repeated over and over. The head of a Death Eater who fell into the bell jar was transformed to an infant version of the man (OP34).
From the Web
Hummingbird on Wikipedia