It is actually a requirement for waterfowl that the eggs' temperature be lowered for a short while each day if you were incubating them. Since she's doing her job instead, that is the time at which the mom would get off her nest to eat, drink, and (more importantly) swim. She brings back moisture in her feathers to the eggs to keep them from a) evaporating too quickly, and b) getting to hard-shelled. Be sure (as always) to provide adequate swimming water so that she can do her job.
Note - if they do hatch and you pick them up to raise them yourself, do not ever let them run short of water. If they do, reintroduce the water to them slowly - a sip or two then a break for a minute, then three sips etc. You'd be surprised how quickly they can spill their water as they play with it.
If you allow her to raise them, expect losses - it's part of nature. They usually have many babies to cover for the loss of a lot, and they're not as good at parenting as geese are.
I had ducks a few yrs back (peking ducks) and they didn't sit on them all the time either, and some of the eggs still hatched (guess the fertile ones). Good luck with them.
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