I don't know about your chickens, but one source for my chickens is wild MICE that move in to their henhouse.
OF COURSE I know that isn't the best source of protein for them to eat.
And yes, I do keep their coop clean, and try to keep the mice out. I have a Tin Cat that I bought from Lehman's (a metal trap that is safe and easy to use and will not harm the chickens in any way), and I catch a lot of the mice using that Tin Cat.
And I have put glue traps in their feeder trough after they have gone to sleep at night, and gone out there to gather the glue traps up before I go to bed. That catches some of the mice, too.
But my house is right near a community drainage ditch, and the coop had to be built less than 20 feet from that ditch. And with the nice, warm environment inside the chicken coop and all that yummy food, the mice move in faster than I can exterminate them.
But my chickens are very happy to help keep the mouse population down by catching -- and eating -- the rodents.
I wish it were different, but I guess it's just another form of food to them...