"Of all the means birds use to talk to one another, the one that most delights mankind is the bird song, which sounds like a beautiful melody, but which is really the way one male threatens another; the more often the male sings his song, the more menacing his warning becomes.
"What the singing bird is doing is telling all other males that he is the owner of a piece of property -- a feeding and nesting ground -- where he shelters his mate and finds food for his young. Should the song of warning be ignored, the invading bird -- who may be attempting to steal the mate by forcing the 'homeowner' out -- is attacked. Usually, however, the threat is enough to scare the challenger off." -- Roger Tory Peterson