You and the other commenter are right, I don’t expect birds to be like “let’s both chirp and make a minor third interval”. I’m sure if they harmonized it would be for whatever sounds right based on how they evolved and learned.
What I’m getting at is it’s not harmonizing if they just happen to be making sounds at the same time. For the purposes of my question harmonizing is when we’re intentionally making two distinct tones simultaneously.
So two birds that are just chirping along their individual songs and they happen to overlap so they each chirp complimentary notes isn’t what I’m interested in. I want to know if any animals mean to harmonize.