Tess,
I have no clue.....I guess it depends if you are trying to breed toward the standard of perfection(SOP)......I am sure there is someone who can answer that better than I can. I went to a lady's house thinking I was getting splash coppers marans roosters for injection blue into my black coppers, and ended with 3 splash rooster, 2 blue birchen pullets, a splash pullet, and a (black)birchen pullet.
I probably have more to learn than anybody around here about birchens. I do like their coloring, though. Who knows what I would end up breeding any of them to my black coppers???