Nubie With Questions About Health of Birds

[ChickenNewbie8]

In not quite sure if I'm posting in the right place but I'm looking g for help with my Rhode Island Red. We have 2 hens about 1 year old and one is full and dark and beautiful and the other is a bit scrawny and her feathers are super light colored, some even have white tones to them. I can't figure out what's wrong with her and am looking for any advice. We also have 2 Barred Rock hens who are doing fine but they both have small bald spits on their backs near the tail. Any thoughts?

 

[cmom]

Do you have pictures? Also do you have a rooster? A rooster can wear the feather off of a hens back with excessive mating.