Research biologist Chris Clark from the University of California at Berkeley has been studying the courtship flights of male Anna's hummingbirds. Using high-speed video to study the mechanics, Clark believes that the males aerobic courtship flights subject them to the equivalent of sustained accelerations that would cause fighter jet pilots to pass out from g-force stress. Using female Anna's decoys, Clark persuaded male Anna's hummingbirds to drop rapidly in thei U-shaped flights that are typical of their courtship displays. For their body size, according to Clark, male Anna's are the fastest moving vertebrates.
As the males accelerate towards the ground, the travel almost 400 times their body-length every second; that's almost twice the speed of the much larger diving peregrine falcon. As they near the ground, the male hummingbirds spread their wings and tail feathers, so that they are pulled upwards into a U-shape glide, at which point, Clark says that the hummingbirds are experiencing 10-Gs; fighter pilots can pass out, or become temporarily blind when they pull 7 or more Gs.
Clark's research was published in the Proceedings of the Royal Society B; you can read more about it here, as well as some of Clark's other research about the chirps produced by the males during flight. In fact, the chirp of the male Anna's tail-feathers as he descends is one of the signs of sping in the Pacific Northwest, as is the angry buzz as one male attempts to ward off another from "his" territory.