In answer to the original question regarding the best size of bumpa bits for chickens, in case anyone else asks the question... I've used large (35mm) bumpa bits on regular hybrid chickens (ISA browns or warrens) to good effect. Last week I got some small & medium ones to try. The small ones are totally useless on ex-batts (Warren or ISA Brown type hybrids). A hen wearing one of these had no trouble plucking one of her flock mates who wasn't well enough to get away. Mercifully I was on hand to intervene... So I'll be trying the medium size when one is next needed.