I have to disagree with you on the parasite side of things. Yes this can be a factor, but not the ONLY reason why they lift.
All birds and nests should be dusted/sprayed with the right chemical before placing it in it's quarters to sit out the clutch. Sometimes, they just have enough, or they are young hens and not cover all of the eggs ect.....
Yes your right, I've will make them double if need be, but that will be it.
Didn't mean it as they only leave a nest because of parasites.I said most of the time they lift through no other reason but parasites in the nest.