Are they bugs or lice nits at the base of the quills? If you have bundles of eggs at the base of the feathers you may have lice. If you separate the feathers to look at the skin (you have to be fast here) you can see tannish bugs with long bodies running on the skin. Sometimes you need to look in a couple different places. The tend to hang out around the vent. DE will help to smother the adult bugs on the birds but won't get rid of them completely. Sevin dust would also have to be reapplied to the birds after 7-10 days to get eggs that have hatched since the last dusting. It's really a never ending battle. I dust my birds every week with DE and keep DE in their coop for dust bathing. Bugs can be brought in by wild birds and hide out in ever little crack you can find. In my opinion the best solution, whatever the bug, would be to hit them hard with a pesticide then keep up on the DE to limit re-infestation. Keep in mind that when you use a pesticide you have an egg withdraw period where you cannot eat any eggs from the chickens. I usually give at least 2 weeks before I eat any of the eggs. Best of luck to you!