Butchering Coturnix and black abdomen

[Tabasco Jack]

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?

 

[quail arizona]

Igbo dove hunting and saw this in a dove breast once

 

[dc3085]

How old were the birds?

 

[Tabasco Jack]

6 months