I've seen two varieties: the jewel-toned ruby-throated and less-flashy black-chinned. In late fall, they fly across the Gulf of Mexico to winter in Mexico and Central America, though I find it remarkable anything that small and that uses that much energy just beating their wings can fly even a couple of miles.
They are quite aggressive defending their food source. Lots of aerial battles take place around the feeders. Eventually, though, everyone gets a drink.