I'm with Misso...it's an AY (heavily marked). My Uncle had a similar story with a pair of "normals" that have just thrown out their first recessive mutation (White Breast) young after almost 30 young. He is 100% sure of the parentage as they are the only cock & hen in that avairy...so it does happen with receissive mutations that after a heap of young a recessive bird is produced even though the statisticians on this forum would argue the chances are 1:4 to produce a colored bird from a pair of splits.
A nice looking bird...pitty you don't know who the parents are as it would be worth while crossing it back to mother/father.
PS: You now have a heap of possible split AY sibblings on your hands
cheers
Paul