If the chickens have the opportunity, they will eat the mice. You might invite a cat to live with you! I try to avoid putting smelly stuff from the kitchen in my compost. No meat products, no dairy according to the compost gurus. IMO, critters are a part of life. Mouse poop = more manure. Seriously, I wouldn't invite wildlife into my compost pile, but it's inevitable that the local wildlife will occasionally go shopping in your pile. Just consider that they are turning the pile for you. It's also a way to be aware of what kind of wildlife is wandering through your yard. When I see that my pile has been dug into by a mammal, (and you can tell the difference between the digging done by a dog or a coon, compared to what your chickens might do.) I know that I need to be extra diligent for predation. If I do have smelly stuff to dispose of, and want to be sure the nutrient value goes into my garden instead of to the dump or septic system, I dig a trench in a garden path, bury the stuff, throw on some mulch, and a bit of chicken wire to keep it from getting dug up b/fore the microbes get to work it over. Again, I think people take compost far too seriously. Especially re: the meat and dairy issue. When my cat gets carried away with his hunting, I either bury his left overs in the compost or the garden. I have no concerns about disease pathogens once they get into the ground and the microbes get to work on them.