I did a little looking into it, and I think this would make a good post since it’s a bit complicated, so it would make a good Owl-natomy post, which I haven’t done in a while.

Bird have a very wild respiratory system due to their energy requirements, so they have very different parts than we do. Short answer is they have tremendous air reserves beyond just their lungs, and their version of a voice box is set much deeper in the chest than ours.

Also due to natural selection, since they use their voice to defend territory and attract a mate since they are solitary animals, louder voices will increase their ability to do both those things.