sorry typing quickly while trying to keep up with the other replies. I was meaning to keep them separate from YOUR flock until you knew what was wrong with them. Now that I see it really is just lousy nutrition and probably mites, it won't be necessary UNLESS there is a big size difference...
Most likely poor nutrition. Scratch isn't enough, particularly when they are not allowed to free range. Lack a source of protein. You could take them are try to isolate them and/or start treatment for mites immediately. Their poor nutrition isn't contagious to your other birds.
Shame on your...