And those pekins in the photo with her? Could they have been involved? Sperm from a male duck can live in a female for up to 3 weeks. I've even heard of cases as long as 6 weeks. Are you sure she wasn't with ANY other ducks for at least 2 1/2 months before this baby hatched?
Your hen...
Okay, now we need to see a photo of momma duck.
Your baby looks like a "buff", but that would require both of the parents to have donated a recessive "dusky" gene, which may not show in either parent, since it is recessive.
And a blue-dilution gene, which your Saxony has, but would also...
Okay, that is a Saxony drake. But that still doesn't explaine your yellow duckling. Is there ANY other shade of color on it? If not, then it will be white, and that can only mean that both parents are carriers of recessive white genes..... or she had a one night stand with a muscovy.
What...