"Bill half an inch. Tail half an inch. Head and body three quarters of an inch. One could hardly believe it to be a bird, and how this minute creature lasted out through severe ocean storms..to Germany, puzzles one.

H.D.A
"(Notes) Humming Birds"
Avicultural Magazine
1914

The Bee hummingbird can store very little energy in its diminutive, but comparatively heavy, vertebrate body. Females are slightly larger. It is thought that a bird any smaller could probably not eat enough to find the energy needed to feed. As it is, these tiny birds must feed almost constantly in order to maintain basic metabolism.

Note: This is the smallest bird in the world and is on the endangered species list.