Tufted Duck ....... variation on a theme ......

A few days ago I posted an image of a very elegant female Tufted Duck, subsequently I went through my picture archive for the species and came across this image that I had never really focused on before.

It appears that there is a variant within the breed that has female Tufted Ducks with a white bill base and bills much darker that the accepted blue bill.

According to my research 5-10 % of female Tufties present with this plumage variation, if anyone has any additional input on this subject please feel free to leave a comment.