I don't think they are stressed, except for that broody is taking up their preferred box! I have seen them stand outside and cry waiting for that particular box, lol.
There are not really any predators in this area. I live in a pretty urban area and I have 3 dogs that would prevent any random animals from attempting to get into my yard. In addition, I have the entire coop/run fenced with 1/2" hardware mesh, so it would be really unlikely they could get in, especially without me knowing.
I will throw some oyster shell out there now. I have it, I have just been lazy about adding it.