Yeah, true. But I've seen three bird calls that don't fit that pattern in my garden. Long tail tits seem to move around in small flocks, probably eating the insects in my trees. Every few seconds one of the flock flies to another tree. If the others don't follow, it flies back. While this is happening there is a constant chorus of little chirps. My guess is that they feel "attracted" to the centroid of the chirps, as a simple, almost mechanical way to keep the flock together (its hard to see each other amongst all the branches).

The other case is juvenile magpies. As adults I mostly hear them shouting aggressively at each other. But if the youngsters don't know I'm there, I've sometimes heard them chatting. The sounds they make are complex and quiet. A bit like this https://www.youtube.com/watch?v=8P4ZVgjslVQ but I've always seen them in pairs or groups when doing this.

Third is the Robin (European). They also have a whisper song. Sometimes they do it so quietly that I have to be within a few feet to hear it. And there's no other bird around. I guess it's a male who is just working on new material to impress the ladies with later.