Hens leave their nest every day to eat, drink and poop. There's a study that cooling eggs once a day can mimic what happens on the nest and improve your hatch rates.
Brinsea has that cooling feature as an optional setting on their incubators. I haven't tried it yet though.
Not necessary true, I have four hens right now sitting on eggs, TWO hens in each coop, when one gets up the other takes over, the eggs are NEVER left alone, never left for "cooling" down. I have even see them Rotate from one nesting bed to the other. Yes, One hen will keep all the eggs warm even though they are in different nesting boxes..
SILKIES are dedicated mommies and once they sit they don't get up. I have had a silkie die sitting on the nest, she refused to get off the nest. I have had a hen sit for 42 days, to hatch out a baby, she was NOT going to get off the nest until then. I finally had to hatch out a baby FOR her.
so I don't believe it's really necessary to cool them down. Don't get me wrong I have heard some people will do it and I don't Dis-them for doing it, all the more power to them. But from what I have observed and seen with my flock, I don't see the need.
I should clarify,when I say "hens", I was talking about my chickens, as far as ducks, again, i've seen them bury the eggs with ANYthing around them, leaves dirt etc.