This is actually for ANY bird. The back is important in that it holds the bird up. The longer it is, the longer the entirety of the bird. If the back is supposed to angle, if the back should be flat, if there are bumps in the back (roach back) or if the back is too short it can cause issues in the breeding department. A nice, long and BROAD back is a sight to behold. You do not want a back that is nice and wide and then all of a sudden becomes narrow and pinches at the tail. You want a bird that has a back that maintains an even width throughout the back. This will cause the hip bones of the hen to be wider and allow for a larger egg as well.