This may sound dumb coming from someone who has hatched numerous chicken eggs ... About what day can you tell for sure that a duck egg is fertile? I can tell that a chicken egg is fertile by about day 3 or 4 because it takes on a pinkish hue and the air cell stands out clearly. I did my first ducks eggs about two years ago (Pekins), so I don't remember much about them other than it took 28 days for them to incubate. The ones I have now are not as big as Pekin eggs, but I can't tell for certain whether or not they're fertile because they were shipped eggs, so the air cells aren't in the best condition and the shells are kind of thick. I know with a chicken egg, the yolk kind of settles toward the pointed end and you can see definite layers in the first week or so, along with blood vessels and sometimes the embryo. Common sense tells me I should be looking for the same thing with duck eggs, but I just wanted to be sure.