Hello! According to the ABA standard, referring the breast and the breast bone, should be “Broad and full, well developed pectoral muscles, carried high.” For my birds, you can’t feel their breast bone, unless you are really trying, the muscles should prevent it from being very noticeable. Lack of this muscle/broadness might also not be because of the bird’s type, but due to the type of feed not being suitable, or a type of worm, making them thinner, and taking away that broadness. It may also have to due with age. Hope this helps!!
