I do wonder about that sometimes. I used to have this beautiful cockerel, he was a mutt, but white all over except for this little black feather on his chest. He was quite tame, being a single child he got a lot of spoiling and attention from momma hen and momma human. One day I looked at him and said "Come here, let me pluck that black feather". I haven't been able to get near that cockerel again
On Monday this week I fed the flock some treats and noticed my little banty, Chooks, wasn't with them, so I told the cockerel "Call Chooks, call Chooks!" And off he went, clucking and calling until she came running. It was the cutest thing.
I think sometimes, maybe on some instinctive level, they do understand what we say, or mean, or maybe they are like dogs, who learn to recognise words and short sentences?