I first started noticing these ruby-throated hummingbirds (Archilochus colubris) feeding on a large patch of jewelweed (Impatiens capensis) in early September. This species of hummingbird beats its wings about 53 times a second and weighs between 2-6 grams (for comparison, 2 nickels weigh 10 grams). You can read more about hummingbirds here.
Check out this computer simulation of a hummingbird in flight, the red indicates vortices of air. Haoxiang Luo, a professor of mechanical engineering at Vanderbilt, built this simulation from slow motion videos of the hummingbirds flying. This cool science blog from NPR talks more about this simulation and how hummingbirds fly.