Hello my natural friend!!!!
What a great article. Read the whole thing it was so interesting.
I would still say that the vocalizations of animals and birds are so primitive, that they can only be considered the very beginning stages of syntax, and not talking in the manner that we usually think talking is, aka like humans talk. I propose that this is not because they are unable to use syntax, as your study shows, but because they are too limited in the sounds they can make. Human vocalisms are far more numerous, meaning the combinations are virtually unlimited. Add to that the increased COMPLICATED syntax we have by creating whole complex sentences due to our increased intelligence.
There are certain exceptions to this rule. Comes to mind a particular parrot that learned to not just mimic human words, but could understand human speech and use it to communicate. That's true language. Also comes to mind apes that have learned with human help non-verbal languages.