I'm of the opinion (just from my own observations) that it may be a sign of blood collecting inside the shell at that area. Like as if they have scratched a vessel in the membrane while trying to kick themselves into position for hatch. I have no idea if this is actually the case, it just...
It isn't yolk sac breakage, unless more than one thing can cause them to appear. I've had it happen twice (once in each of my duck hatches). The first one died in shell, but it was fully formed with the yolk sac still intact and partially absorbed. When I eggtopsied, however, there was a lot...
Ok, that pic is really small and my eyes aren't working so good right now. Do the eggs kinda look like they have become "bruised" in spots? If so, I have hatched ducks twice now, and have had it happen both times. The first time, the baby died in the shell, but the second time it hatched and...