i have 7 chickens all 1year 5months. around the end of may thier butt feathers started falling out and thier butts got red. we thought they had mites so treated with a pyrethrin spray. treated them and the run and coop. we have been doing this once a month. still having the same problem, not even sure if it is mites. any help would be great. they have never stopped laying, and eat just fine.

Yes, mites are a ongoing problem with chickens. I don't think you can ever fully get rid of them, but more keeping them at bay. Everytime you clean your chicken house you could use some lice/mite powder on it, and about once every 6 months powder all the chickens. Good luck :).
Diane knows quite a lot about chickens so she should be able to help you :).

That is almost certainly lice, probably the common fowl lice.

Here are some pics that might help from when ours had the common fowl lice, the lice themselves were too small to get any decent pics but you can see the effects on the feathers around the bottom area and the lice eggs really clearly.







To get rid of these lice you can get various powder from poultry supply shops and just follow the instructions, we also trimmed the feathers at the bottom and they healed good as new.

Also have a look at our article:

External parasites in chickens

thank's guys so very much, that helps and makes me feel better. one more question. i do not know if you guys have fleas and ticks way over there, but if you do, can they get on chickens? this summer has been really bad for us with the fleas and ticks.  

Yes we certainly do have fleas and ticks, luckily our chickens haven't had them so far just fowl lice and red mite, red mite is the constant scourge here that is almost universal in any wooden housed chickens and needs a lot of work to stay on top of.

Quoted from coyote

thank's guys so very much, that helps and makes me feel better.

I know I was really shocked when our chickens got red mite...then scaly leg mite...then poultry lice...felt like a really bad owner :(.  But, in reality it seems that these parasites are an unfortunate downside of chicken keeping.  All you can do is be vigilant, keep the house as clean as possible and use preventative measures and early treatments to control these problems.