how confident are we that they don’t? I hear a song sparrow and I can’t help but think their calls sound like compressed data, almost like a modem
Take ravens for example. There's one that hangs out in trees directly outside of my house now and then. It makes new sounds I haven't heard before quite often. I don't hear other ravens, so I'm not even sure it's trying to communicate. Does it have a social purpose? Is it bored? Do the variations express anything at all?
Yet being in a forest with them around, you hear all kinds of noises too, and they do it all together. They interact a lot. Though my experience is that they use fewer variations when they're together compared to the weird one outside of my house.
They seem very intelligent regardless. They're such cool birds.
Meanwhile most of the other birds around my house, as far as my ears can tell, just make the same sounds over and over. What does any of it mean?
As birds spread to fill many different niches, their vocalizations had to diversify so they could still find each other.
Complex mating displays (singing, dancing, nest building) are fitness demonstrations. They correlate to endurance, resourcefulness, memory, etc.
And of course, some behaviors may simply be vestigial.