I just killed about 50 of those guys. Took your advice and let maybe 45 of them sprout their wings. But the last one on the bridge I really wanted to shove my halberd through his back. And he's the one who dropped a Great Corvian Scythe for me. Of the 45 who sprouted wings. 2 gave me Corvian Greatknives, and 1 dropped a staff. Also ate a couple rusty coins, and no one dropped anything while those were active.