Black spots on egg yolk

[patmas57]

Hi, I've just rejoined the TPC after nearly 6 months of absence. I have a question for the experienced here. I took my two ducks, a Cayuga and a Golden Hybrid 300, through a forced moult this spring (once it warmed up enough to do so). They weren't happy about eating rolled oats during that time, but otherwise no problems. They started laying again about a month ago, and I've noticed that the Cayuga has tiny black spots in one location on the yolks of her eggs. I made a birthday cake for my husband last week, and tossed every other egg because of these spots. Yesterday, after checking a few, I just tossed out all her eggs. Since she's a Cayuga, I can tell which ones are hers from the grayish coating. But I'm wondering now why these spots? I'm not taking a chance on eating the eggs, and we get one a day already from the Golden 300, so it's not a great loss, but I'm concerned that she has some sort of illness. Otherwise, she's healthy and apparently happy. Does anybody know what the black spots could be? I do also have a blood spot once in a while, but they're always red. Both ducks were hatched out April 14, 2008, and purchased from Metzer Farms.:|
Thanks for any advice, Patty