These are Ruby-throated hummingbirds (Archilochus colubris). The one on the left is a female and the one in the picture on the right with the red throat is a male.  If you are using 800x600 screen resolution on a 17" monitor or 1024x768 on a 19" monitor (1280x1024 on 21" monitor etc.), these are larger than life.  That should give you an idea of how small they are.  I took these photos by sitting motionless next to the feeder for several minutes and waiting for the hummers to arrive.  This is an excellent exercise in patience.