Reminds me of a recent post about a visible air cell on a duck egg. I’m pasting the link below. I thought that blood might have gotten into the air cell, darkening it?
https://www.backyardchickens.com/posts/21480020
Just looked at the image of the duck egg in the link. Very interesting, I noticed this with the lone duck egg I currently have in my incubator. I don't know what causes it but it has only been incubating for about a week and it doesn't seem to even be fertilized to me so I don't have any idea what would make the air cell line visible like this.
I'm going to wait until next week to ditch the egg but it doesn't seem to me like this will not be my year for duckies since the entire first batch of Mallard eggs was no good either.

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