What the fuck is up with that Roger Waters scream?

Seriously, what is the deal with that sound that Roger makes with his throat? You know, the one that sounds like a bird call, like a rapid succession of high pitched, other-worldly noises. He did it a lot more in the early years, like in "Careful with that Axe, Eugene," but he does do it on the Wall once. You can hear him do it 1:00 into this version of "Run Like Hell," and again 5:00 in. Though the best example of Roger's bizzare vocal abilities would be the Ummagumma track "Several Species of Small Furry Animals Gathered Together in a Cave and Grooving with a Pict," which is a vocal collage of his ravings sped up in the studio to sound like small furry animals. The noises that this man emits are freaky enough as they are, so to modulate them like this makes for one unnerving listening experience.
I can't really think of a vocal trait in rock any more distinctive and quirky than this screeching that Roger does.