I saw a hummer fall from a feeder about 30 feet from where I was sitting; I did not see another bird as the culprit. I thought it might be a bumble bee and walked over to look; it was a hummer. I was looking down on him wondering what my next move was and hoping he would fly. He did fly away after a few seconds. A couple of days later (yesterday), I saw 2 hummers go to the ground. One did not get up and the other bird came back and appeared to peck him a couple of times. This was about 15 feet from where I was sitting. As I got up, the bird on the ground flew off. I have a lot of juves now and I am sure they don't know all the rules. I have seen this "taking to the ground" 4 or 5 times over the years (about 25). The aggressor can kill the bird on the ground, but I usually run them off and let the one on the ground the opportunity to fly away. Of course I am not always there.