Even that's not that clear cut. The mutants and the nonmutant proto chickens interbred regularly and different mutants showed up and also interbred. The real answer is there's no platonic ideal chicken but we really want to categorize this thing.

Edit: I guess the platonic ideal chicken is a man according to diogenes.

I have no direct knowledge about that, but if we take the analogy of the egg (shell, albumen and yolk sack) being the life-support system of the embryo during gestation, in humans the placenta would be a big part of that, and exactly whose body it is part of its not simple (from what I remember both mother and child contribute cells, and the 'plan' for building it comes from the father's genes). So maybe for chickens it could be ambiguous whether the shell 'belongs' to the laying generation or the hatching one. Seems like mostly a human taxonomy distinction to make anyway, obviously it's in between the two, but we like to draw the line somewhere.