Chick with growth on breast.

[Eggcessive]

What type of feed are you using? A balanced chicken feed for the age is best rather than giving treats. A small bag of granite poultry grit will make sure that they are getting enough grit for the gizzard to grind up what they eat. I had a hen once who ate all sorts of things around the yard, tree algae, dried grasses, and junk, even though there were pans of chicken feed in multiple places around the chicken yard. It is called pica. She eventually died of crop and gizzard disorders, and that is what I found inside her.

 

[Allilang]

They eat layer feed since I moved them in with the rest of the flock. The old hens ate all the chick feed. They are all free range and eat whatever is around, but they especially like kitchen scraps.

Sweetpea is the only one who is having any problems. She is now in isolation and only has access to water, grit and soft food.

 

[Eggcessive]

8-10 week old chickens are too young to have layer feed. It has 4 times the calcium needed (until they are 20 weeks old and laying.) Too much calcium in young chickens can damage the kidneys and may cause gout. Chick feed or all flock feed is fine for all chickens, and providing crushed oyster shell for the layers is much better.

 

[Wyorp Rock]

I'd get a bag of chick starter or all flock feed and provide it for the whole flock. At 8-10wks of age, the calcium content in layer feed is too high.

As for why she's eating feathers and sand, I have no idea. Chickens eat a lot of things.
I'd watch to make sure she's able to get to food/water and is not being kept away. Sometimes that happens too. Adding more feed/water stations can help with this.