Depends on how the mouse died if they killed it dont worry about it as long as it wasnt poisoned your fine. Chickens are very much little meat eaters. They will kill and eat anything smaller then them that includes mice, humming birds, frogs, lizards, ect.
I don't know how it was killed. All I know is we have no poison here. I have found a few dead mice around but I don't know how they died. We have traps out but they are no poison traps as we have children and dogs as well