On first impression one might attribute this confrontation to territorialism, competition for food, some kind of courting joust or ritual or maybe just establishing a pecking order. After observing for a while, however, I'll go with the more anthropomorphic explanation that the bird on the right is just a jerk. He was going from bird to bird, "attacking" them without apparent provocation, didn't seem to care if the victims left the immediate area or not.
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