Hello, and a belated welcome! I'm so sorry for your losses. 

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Apparently there are some from what I've read. Mareks, Newcastle, and others. But that's just what I read. But It could have been bird flu for sure. We also seem to live under a duck migration path and one of my neighbors attracts all kinds of birds with bird feeders and such. One next door neighbor has chickens and they are all fine. Actually where I got the two ducks from. The white one was actually attacked by their dog when he was really young (messed one of his legs up) before I took him over so he is lame but actually gets around pretty good now. Both ducks get tons of attention now where as before they were completely neglected.Hello and welcome to BYC!
I am so sorry for your losses.I don't know of too many things kill birds that fast other than Bird Flu, poisons or Teflon heat lamps. Most viruses and even bacterias wear them down a bit before death, and rarely do these things take out an entire flock all at once. Hopefully you never have to go through this again.
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For about a week only. They were from next door so I was able to see them. Tho the white duck was pretty neglected he didn't seem sick, but had weepy eyes. I should have kept them separated longer.
Hello, sorry to read of your losses. Were the ducks in isolation before they were added to your flock of chickens?