Hence my post, I understand the term cull quite well, but some folks cant bear the thought of giving a bird up for someone else to take, use, eat. The question of how to get to a pure bred sussex through a bird that is not pure strikes me as unique. On one hand I want a pure bred breed on the other hand I am starting with a mixed breed, and to get to a pure bred I need to cull and sort and breed back to original bird and on and on.
Seems like the common sense thing would be to keep the pet, allow her to produce a couple of chicks if that is important but then go get some pure bred stuff. Logic is sometimes the least of the issue in these cases.