My duck will not sit pn her eggs.
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18 eggs is a bit much. Try taking some eggs off, a typical clutch size is about 8-12 eggs. For a duck her size I'd say leave the 10 newest eggs. It could also be that she just isn't broody anymore. I had a duck that went broody once, she only had 5 eggs and she quit sitting on them on day five. Sometimes they just aren't ready. If taking some away doesn't work I'd recomend putting false eggs (like golf balls or wooden eggs) in the nest to encourage her to go broody again. Do this after you remove the eggs she abandoned of course.