I think they keep passing it around just before they die.
try killing a little ways in the zone then work back the front, with luck they won't be able to pass it to the next guy and you'll get it from the mob at the entrance.
I can spend 2 hours in HOH and watch 3 or more Silverwing Loops drop and rot. I spent the last 4 days here for a good amount of time killing all of these that supposed to drop but zippo!...Grrrrrr:( Bristlebane 65 Paladin
I spent a couple evenings killing various mobs, mostly on the near side of the bridge (and including several spiriters), and never saw this drop. Contrast with Droga, where my group of three was leaving Silverwing Belts to rot because we all had them after a couple hours. Am I just having a bad run of luck, or do the faceguards seem to drop more from mobs deeper in the zone?