Blood on rear ends

Chickens are very attracted to pecking blood or anything red for that matter.  Clean off the blood and you may want to remove them  from the coop until they are scabbed over.  There is a product called Blu Kote sold in farm stores like Tractor Supply that is an antiseptic with a blue pigment.  The blue covers up the tempting red color to reduce pecking and makes it safer to put the hens back in the coop once they are no longer actively bleeding.

Pecking is a management issue. When chickens get bored, are crowded, or are being fed incorrectly, they peck each other. Once they draw blood, and once they get into the habit, it is very hard to stop. Do use the blu-kote. If you can identify one hen that is a bully, remove her for 3 days. Sometimes, when you put her back after a break, she'll be lower in the pecking order. Meanwhile, do you have a minimum of 4 square feet per hen inside and another 8 square feet outside? Do you have inside and outside roosts? Feeders that aren't crowded? Places to dust bathe? Is most of their feed intake laying hen pellets? Do they get greens or are able to free-range? Do they get free-choice oyster shell? There are many variables! Once in awhile I've had feather picking here. A little readjustment of the management and the problem disappears, so, it can be solved. Good luck!

I second everything that has already been said!  I just dealt with this myself (it was food dominance, not boredom... result of changing their feed set up) and the only thing at worked was pine tar:

http://www.tractorsupply.com/bickmore-pine-tar-16-oz--5027552

I was paranoid about using it - especially since its smell was so noxious - but it was a miracle!!! Initially I just put it on the bald and bloody spots, but they kept going. Once I put it on the feathers surrounding the targeted areas, they stopped! I also got a flock block, and that gave them a good distraction.  I'll be making my own blocks from now on, it was expensive!  Seriously... get the pine tar and keep it on hand. It was a miracle! if you don't believe me - search the forums, it's been a miracle for so many others.