These only drop off the lvl 10< yags. Once you go through the tunnel in Giddeus you have gotten to the yags that drop necklaces instead of yag feathers. So, most people who go to giddeus won't even notice the drops because they ussually walk past these, since they don't drop necklaces or keys.
Basically they are the same as the ones in sarutabaruta but more concentrated... unfortunately, they are too easy to kill and the whole waiting 5 mins for them to respawn when you can wipe out the entire room in 30 seconds is also quite annoying...
They drop like water (I get 1-3 per yag), which is almost the same rate as the necklace drops off the higher lvl yags. I never go the the castle so I don't know much about the yags there, other then they are higher lvl then even giddeus (start at the highest lvl in giddeus and go up from there) so I imagine that they do not drop feathers as well.
Hope that helps, though this is the first post in over 6 months and I imagine no body really cares.
To reply to the comments below: I'm a THF46, meaning I obviously have Treasure Hunter II. I routinely aid low-level friends in searching for the three keys for the Kazham pass, including the Giddeus key. Never once from any of my expeditions (and my sometimes hours-long farming sessions getting beads for my Utsusemi for about a week straight) did I get a single Yagudo feather off of the mobs within the bounds of Giddeus.
In Sarutabaruta and Tahrongi, however, I get anywhere from two to five feathers per kill. None of the other Yagudo have ever dropped feathers for me.
If I'm walking on foot through Saruta and you want to tail me, just follow the trail of discarded Yag feathers. ;) Seriously, if you can't get Yagudo feathers to drop then you must have some sort of secret Treasure Hunter -10000000 effect on you.
I was planning on making Yew Wands from 22-23 with my woodworking, so that I could finally get a use out of these. I thought "damn, farming these will be so easy." But ironically, I was lazy and just bought them. Of course, that's cuz I was in Sandy for advanced image support, so I had to rely on one of my mules to get them. ^^;;
If there were some sort of special filter to stop this permanently as a drop, i would pay to have it.
Anything past the tier 1 yags and they drop at least 5 feathers, and at the most I think ive seen 9 or 10, that all drop seperately and spam your screen with the worthless things. I imagine that HNM yagudo drops at least 10 hundred million feathers.
Wanted to add something about the drop : it doesn't seem to drop off Yagudos in dungeons (i.e in Giddeus or Oztroja), but only from the ones in normal areas (like Sarutabaruta, Sauromugue, etc).