I am new to chickens, quick back story is I have 8 chickens, 5 red 3 black star. Recently as of today I have noticed that there are 3 of then that have their feathers pecked off the top of their back towards tail and also under chin on neck. They have been healthy and no problems until this. They were born in April 2013. Every once and a whole I have one that gets a little like these but not as bad nor as many. Any suggestions on what's going on and is it normal? Or do I need to do corrective measures. Thanks!