In birds (Parrots specifically), why does speech effect their eyes?

I just recently moved in with my brother and he’s got quite the zoo here.

Jay Jay (their macaw), took a little bit to warm up to me, so he was a little shy at first. Now, I hear him speak all the time. I’ve noticed that the iris of his eyes contract a high degree when he says something. I asked my brother why they do that and all he said was, “It takes a lot out of him to speak.”

I know there’s got to be a more scientific or biological, reason and I was just wondering why. Care to share your thoughts?