Welcome to BYC! The testicles of a duck are well up in the body cavity next to the kidneys, and that sack is attached to the penis. I guess all I am trying to say is that it's not the intestines nor is it a testicle.I'd recommend a good cleaning daily, or twice daily, a mild soap. It looks like a twisted testicular sack. However, the intestines are right there too. If you are able to get the sack to reduce it's swelling it may go back on it's own.

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