At this time of year I hear Great Horned Owls squawking/shrieking as much as I hear them hooting. The shrieks are usually thought of as a juvenile call but I've read that they are also used by adults during courtship.
This morning I came upon an owl that shrieked continuously for over half an hour, and it must have been calling well before I arrived.
The calls vary in amplitude as the bird turns its head (6.29am, Dec, PA).
Towards the end of the recording you can hear a second owl whistling in the distance. On hearing this other bird, the shrieking owl flew off in the direction of the other owl. From then on the two alternated calls, the second owl calling with higher shrieks and whistles. As female owls have higher calls, I suspect the second bird was a female, the first bird a male.
Whistles and high shrieks by the second bird, beside a creek (the first bird was also calling, but is barely audible, 6.37am).
High shrieks alternating with low shrieks in background (6.50am).
Low shriek by the first bird, followed by whistle by the second bird (7.02am).
The calls continued to about 7.10am, just before sunrise. Based on the time of year, which is shortly before egg laying time for Great Horned Owls, I suspect that this was an adult pair, communicating with shrieks instead of the more familiar hooting. But, unlikely as it may seem, I can't rule out the possibility that one or both birds were lingering immatures.