I've seen this before in my own birds. I have not had a rooster here since he declared himself in midsummer, but in October and November, we got an egg or two that looked like this. A sort-of bullseye. I had read the thread regarding the appearance of fertile eggs, and am still somewhat confused. We have one undeclared bird. It is a Barred Rock, and larger than the other BR. Smaller comb and wattles, and had never laid an egg. We actually heard crowing one day, but couldn't identify if it had actually come from that bird, or not. We put the two BR's in the tractor, alone, and voila! two eggs that look slightly different colors. No eggs since that we can attribute to that bird, though, no crowing, and no more ?fertile eggs. I guess that it could be non-fertile. Just a slightly different-appearing non-fertile. I hope someone comes along who is an expert at this.