There was an owl hooting outside our house earlier, and it occurred to me that every other bird has a high-pitched call.
Ravens have a croak that could be considered low, but their loud call is a caw that’s higher. I can’t think of another bird with a call nearly as low as owls’.
Search engines are no help, mostly duplicates answering why they hoot. Why are owls’ calls so much lower than other birds?
The syrinx is a birds equivalent of our vocal chords.
From Wikipedia
Addendum from PA Game Commission