@johncarlosbaez Would I be right in saying that many birds never encounter polyphony*? There must be some that sing together, I guess. I wonder if there's a connection there. Strict rules about harmony are only really necessary if you are modulating keys a lot, or doing polyphony - whether unison, or harmony.
*edit - I mean intentional polyphony. As opposed to the cacophony of birdsong I mention later.