We noticed today that some of the hummingbirds in the back yard tended to chase off others who approached the feeder. I saw one who would sit on a branch at the treeline (woods begins ~ 35’ from the back of the house) and go after some other hummingbirds, while allowing a few to take nectar from the feeder.
The Wiki article offers this:
That doesn’t really explain the behavior observed. I know we have some bird wise souls around here, so over to you.
Is there a 4th of July ornithologist convention drawing away all bird knowledge? 
Hey, I’m just emerging from my overnight torpor. 
My thesis was actually on territorial behavior in hummingbirds. In some species, individuals (especially males), will defend patches of flowers in order to guarantee themselves a continous supply of nectar. They will adjust the number of flowers they will defend based on the nectar production rate of the flowers. Larger, dominant species have higher energy demands and will defend more productive flowers and larger territories. Some species do not defend flowers, but visit more dispersed or less productive flowers which it is not economical to defend because of the energetic cost involved in chases.
Hummingbirds are unable to recognize that a feeder provides an essentially limitless source of nectar. They will defend it even though there is really plenty for all. They may “allow” others to take nectar in certain cases, for example: 1) it is another dominant individual that they are unable to displace easily; 2) it is a female and they want to keep her around so they can mate with her; 3) they happen to be full and/or tuckered out and are not sufficiently motivated to chase off intruders.