What a dreadful thought so let’s address that straight away. Passerine birds can perch. Robins love to ground-feed. They sing when doing these things and are often on or near the ground. So they would have to be pretty darn traumatised to carry on screaming.
Of course swifts are different. They cannot perch and thus keep flying all day (even the nest boxes we put up have to be specially designed in a vertical manner to help them use them). They may therefore be screaming that they want to stop…but perhaps they are screaming like a kid at a fairground because they “just wanna go faster” !
I prefer this idea. I think they are singing the bird equivalent of “wheeeeeee” as they swoop and dive and frankly, why wouldn’t they ?
This does all beg an interesting question : like a human child, has a baby bird ever said “I don’t want to learn to fly” or “I’m too tired to catch food” or “You just don’t understand me, I wish I’d never been hatched” or is that sort of self-indulgence an inherently human one ?