Flock calling is different from screaming.
Flock calling is just keeping in contact with a flock mate, checking up on them when you can't see them...
A LOT of conure screaming is actually, I can't see you, so I'm gonna check on you...
There's the alert screaming too. When something is spotted outside. I'll come up to the cage or perch in room , depending on where they are and we have many windows in this house so the birds are always able to see outside.
I'll lift my Zon's up and Poppie usually say's "look look, what's that?" and I'll have to explain it to her that it's not going to hurt her and that it's ok. Such as a squirrel in tree, or rabbit or deer in yard. It's a Zoo outside actually so the birds see many types of different animals.
Yesterday they all created ruckus when a couple Turkey buzzards swooped low across the yard. Want to hear some screaming...man I need ear plugs for the " Big bird in sky alert".