thanks everyone...
in the vet's defense, this is farm country and culling sick/injured animals is a fact of life (just one I'm a failure at).
No camera here but this is a description of the absess/boil:
round shape under the skin at the back of her head - approx the size of a very small banty egg or the size of an unshelled hazelnut. It is solid to the touch and unlike a blister. When lanced, no discharge except a bit of blood from the skin. The condition came on very suddenly - not there at the a.m. feeding then appearing that evening.
the hen has maintained a solid weight, has good color, is functioning normally in all respects - no apparent distress or associated problems noted - none of her coop mates are ill or showing symptoms of any kind.
no sign of a tick and cannot determine if there is a spider bite. Have added DE to the ration for the last week. Have treated the hen with antibiotic cream and some blue kote (although no one is picking at her).
This looks like a boil in humans. At this point I'm taking a wait and see attitude. I'm unwilling to cull her as long as she's doing ok. I've talked to more experienced neighbors and both claim they've never seen anything like it before.
thanks all for your support
Catherine