I have just one hen (of 14) that has this ongoing problem. She acts fine. Eats well. Free Ranges far and wide with all the other chickens. Lays an egg every day but often airdrops it from the roost and it ALWAYS has a gritty paper-thin WHITE shell rather than the brown it used to. She always has dirty butt-feathers and mild chocolatey diarrhea. This has been going on for months. Again, she acts healthy and happy. What should I do???????