Pekin Duck Dies
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I don't raise ducks but I noticed that your post hadn't been answered yet....so I am guessing nobody has an idea of what happened. I don't think falling into ice cold water caused damage though as I had a two week old baby turkey that kept jumping into my toilet bowl. The cold water could have killed him and he was very unresponsive as I fished him out. I had to turn on the incubator and put him in for a few hours to warm him up.
Had the duck been eating and pooping well before its demise? If you didn't notice any signs of predators or illness, perhaps it just wasn't healthy to start with. I have had turkey chicks die at 3 months for no reason....they just appear dead in the coop the next day....but they were always on the small side....so I came to the conclusion they weren't thriving for some physiological reason.
I think you should find some company for your other duck.