Breeding season has begun in the southern climes of North America, and birds are defining and defending territories. If you want to know where a mating pair's territory begins and ends, playing a song of a local species can tell you very quickly, as this bird language article details. Just this morning I tried that tactic in my backyard on a couple of Carolina wrens who were on the fence nearby. Within five seconds, both wrens flew to within ten feet of where I was sitting, the male with his incredibly loud voice box singing to the heavens, and the female "chattering" incessantly, both letting me in no uncertain terms that this is their territory and that I need to leave...NOW! I turned off the song on my smartphone (something you should only do sparingly...no need to unnecessarily stress out our avian friends, they have enough challenges to face already).
As I went back inside I saw another dispute through the window, this time between a female cardinal and another female...or at least she thought so. Unfortunately the window had a reflective covering on it, so the female was responding to herself, continually ramming the window with her bill and using her feet to try and dishevel the feathers of her adversary. In bird language terms, this type of behavior is one of 'The 5 Voices' called territorial aggression and is most readily apparent in the springtime. As the story above illustrates, it doesn't only occur among male songbirds, as females have their own squabbles. So over the course of the next few months as the breeding season gets into high gear, keep an eye out for territorial aggression as it is a great teacher of bird language. If you pay close enough attention, you will begin to learn for each species how to distinguish it from alarm calls (one hint: if only a single species is demonstrating this type of behavior, it probably is territorial aggression rather than an alarm call).