Song Sparrows Communcate
with Visual Displays
Birds and other higher animals communicate not only with sound, but also with their bodies.
For instance, if two males are confronting one another at the boundary of their side-by-side territories, they may raise their tails and lower their heads so that their entire bodies assume a more or less horizontal position, and they'll fluff their feathers, making themselves look larger than they really are. This "puffed-out display" can be translated as, "Don't you come another hop closer to my territory because I'm big and tough and I'm ready to fight!"
If one of the males decides that the other is bluffing, or trying to grab territory that doesn't belong to him, the challenging male may not only fluff up his own feathers, but also, while singing, raise his wings above his back, and possibly even vibrate them. Of course all this commotion lends him an even more formidable presence than his merely "puffed-out" neighbor. This "puff-sing-wave display" can be translated as, "Oh yeah? Well, I think I have as much right to be here as you do, buddy, and if you're so interested in fighting, then I'm ready, too, and I'm even bigger and meaner than you are!"
This form of communication is so effective that only seldom do Song Sparrows actually fight one another, and even that is mostly an exercise in wing flapping and tail flicking -- though sometimes feathers do literally fly...