You're speaking with Dr. Michael Salkin. Welcome to JustAnswer. I'm currently typing up my reply.
I'm sorry to hear of this with your duck. Ducks are particularly susceptible to certain toxins and his symptoms - particulary his floppy neck - suggest botulism toxin ("limberneck"). Affected ducks usually die in a coma within 24-48 hours. We rarely attempt to treat these ducks due to the grave prognosis upon presentation.
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