Bird songs and bird calls are not the same thing, even though the two words get used interchangeably all the time. Songs are longer and more musical. They're given mostly during breeding season, usually to hold a territory or attract a mate. That's traditionally been described as male behavior, though female song has turned out to be far more common than birders assumed for a long time. Calls are shorter and simpler. Both sexes give them year round, and they do practical jobs: staying in contact, sounding an alarm, keeping a flock together in flight. An American Robin is the easiest place to hear both. The cheerful, rolling warble is the song. The sharp, single-note chirp when something startles it is a call. Same bird, two very different sounds doing two very different jobs. This is often the first step in learning birdsong, or I guess sometimes, birdcall!