Coccidiosis... internal parasites or I think the correct term is protozoa... that takes time for the bird to get immune to. It's something that is in the litter, soil, grass, and other areas. Best bet is to use amprolium for the first two weeks or medicated chick starter until they build up an immunity... after two weeks they will be fine.
Not usually a problem but if they are over crowded and in very damp conditions it could make a bad dream turn into a nightmare real quick....
Bottom line is keep them dry and they will build their own immunity to it over time.
I agree... DRY is the most important word when it comes to keeping chickens.
Now about the feathers... Broilers do grow unnaturally fast, and you will see bald spots. It will save you much aggravation if you allow the bird to fully feather out before you butcher it, or the plucking will be a nightmare. Pinfeathers are NOT fun, especially when the bird is covered in them.
As far as their behavior, a bird's feathers grow inside a sheath or tube, which will flake away as the feather grows out. Birds will use their beaks to strip away these sheaths as part of their normal preening behavior.
When the bird's feathers are growing in, they will preen and fuss over their feathers a lot more than a bird that is fully feathered, but even at the best of times, a great deal of a bird's day is occupied with preening.
btw... it's these feather sheaths that cause a great deal of the dust and dander associated with baby chicks.