Why is a hummingbird a Christian symbol?

It's not a hummingbird, it's a dove. It's because in the story of Noah and the ark, Noah sent another bird to find land, but that bird never came back. So next he sent a dove, which brought him back a tree branch. Since they had only seen water for 40 days, they considered it a promise of new life. The dove is a Christian symbol because it is a promise of new and everlasting life.