I can post a photo later, but the last 2 duck eggs I collected had a blue/teal dot (almost looked like somebody took a fine tip marker and put a dot on the egg shell). It was on the top of the egg as it lay there. Any clue as to what this might mean?
Blue dot on duck egg shell
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Pictures are definitely going to be helpful in answering this for you. In the meantime, can you tell us a bit more about your ducks - what breed are they? What color was the rest of the egg shell? Where are your ducks currently laying - in a nest of their own choosing/design or a nesting area you have provided? Does anyone else have access to your duck's nest? Can the spot be easily removed - have you tried dry wiping, fingernail scraping or washing and, if so, what happened?