overmated females will be more whiteIt depends on duck to duck. I read an article about it recently. Two one year old black runner siblings whitened different amounts. One was about a third to a quarter white and the other was barely white at all. They said it had something to do with damaged feathers. I am not entirely sure what they meant by that.
Cayuga go white with age but I have 2 males who are 8 years old and still totally black, I have a mixed 1/2 cayuga hen who looks a lot like this one though