I beg to differ. Just because an egg is in the nest doesn't follow that it will complete the fertilization process. I've got old hens whose eggs are no longer viable, no more than an older woman's eggs are viable...she can release them all the same but it doesn't mean she can bear a child. These will even show a bull's eye denoting they've been fertilized, but most do not survive to grow into something. There is something about these heritage gals that keeps their eggs from going all the way to completion....they might develop so far and then stop or they don't develop at all, but are clearly fertilized and show it on the egg.
The rooster is doing his job and there are eggs in the nest, but the heritage female is not showing up in the final hatch like the hatchery birds. This tells me something is wrong in there...don't know what it's called, exactly, so I just say lack of actual fertility. Something is wrong with their eggs, in other words.
Come to think of it, they aren't real good layers either, so I guess one could say, in truth, they aren't real fertile.