I have killed many Mallards with the worms. About one out of every ten I kill have these worms. Earlier this season two out of the four Mallards I killed one morning had them. I know it's probably safe eating them, but I just can't make myself do it. Other than Mallards and Shovelers I have not killed any more species that had them. As for the worms in the divers, they are probably ones that have been ingested by the duck. When I was younger, we had some ponds close to where I lived. The ponds were usually full of Scaup. Just about every Scaup we killed, and that was a bunch, were full of these red colored worms that they were eating. It was pretty gross looking, but had no effect on the breast of the duck.