Also, when they are a week old let's do a tail feather check to see which ones have little tail feathers and which do not. My theory so far, is that the girls get their tail feathers first. You can see them at a week old and the little roos have no tail feathers. I did that with my AMs last year and I thought I had 6 males and 9 hens. Turned out I had 5 roos and 10 hens.
I'll be doing a big gender experiment this spring and sharing it on the ABC forum too. I'll add this to my list of possible gender tells.
My list includes: day old: comb size, toe size (old wives tale), leg size, wing pattern; week old: tail growth, wing color, wing growth.
If anyone has other suggestions, I'm all ears!