Why does the hawk screech?

The red-tailed hawk has a great cry, or call, or whatever it is, which is something of a mystery. We hear it fairly often and call it a shriek, but it is generally known as a scream or a screech. You can hear it here:

https://www.allaboutbirds.org/guide/Red-tailed_Hawk/sounds

The interesting thing is that no one is sure why the hawk is doing it. In fact the explanations tend to be contradictory. Sometimes it is said to be territorial, warning other hawks away. But then it is said to be a warning cry, but warning who? Other hawks? It seems to have been observed in both uses.

Another possibility is that it is a sort of a warning to something it might attack, that is a threat of sorts. My wife once came too close to an eagle's nest and was warned off by the eagle screaming while hovering over her. It was very effective.

But recently I observed a hawk screeching that seemed to fit none of these modes. I first saw it flying low over the horse field, shrieking as it went. It then landed in a tree, where it continued to call. This calling then went on, off and on, for over two hours, from various locations.

It is late September so there is no nest to guard. Nor does this seem like driving off a threat, because the hawk was all over the place, as it were. We only see or hear hawks very intermittently, so defending a territory seems unlikely. I suppose one possibility is that this was a juvenile hawk, marking out a territory for the first time, or some such, but that is a stretch since the migration is nigh.

So I am inclined to think that we just do not know what it was doing. The primary study on hawks appears to be this one from 1946:

https://sora.unm.edu/sites/default/files/journals/condor/v048n05/p0205-p0237.pdf

But as the authors point out, this is a very specialized environment, where the hawks are closely packed. Territorial fighting is common, often carried out by mating pairs. We are more at the opposite extreme, in a national forest where the only open ground is the scattered valley farm. We only see hawks from time to time and seldom see two red-tails at once. I doubt there are any territories here, but in any case the protracted behavior I observed the other day was nothing like what the authors describe.

So we have yet another mystery, in this case a rather loud and entertaining one.