I'm culling some layers before winter and some of the hens have a black, discolored abdomen. It's just hens, no roos have this. I've spent a couple of hours already searching for images or descriptions here on BYC and the web and haven't found anything yet. These are about 6 months old, pen raised. About a month ago I switched to Natures Best organic turkey feed. These birds are not showing any outward signs of being sick. The vent was not extended before skinning. Any thoughts?