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Alot of times the first sitters will do just that, They have part of the instinct but not the whole idea. I have noticed some breeds are more broody than others - Hollands, Midgets, and RP's are our most broody in that order. When a Holland hen decides to sit she might as well be glued to the nest.
Some say that an egg has to be at incubating temp for 24 hours before they start to develop. But who knows, if you dig around enough on the net you can find a study to support just about anything. When you think about it, if you have a couple hens laying in the same nest the eggs could be warmed for quite a few hours a day as each hen lays. I would say hatchability would go down over time but not sure if from the warming and cooling or just the age of the eggs. For our hens we mark all the eggs in the nest each day they are layed and once the hen sits steady we swap them out with fresh eggs.
Steve