Sorry it took so long to reply! There is always a pecking order, which means there is always a hen at the bottom of the flock. Hens like to pick off bugs and such off of other hens. All my hens did it too my rooster
The "mud" on her feathers could be chicken poop,
, or mud if it rained or is muddy around, they somehow always manage to get dirty! I am glad that she is taking dust bathes! It keeps mites away and shows that she is content


