Hatchery ducklings from farm store mysteriously dying...

[isis]

I'll start out by saying I've raised baby ****s for years... both ones that I've bought and ones that I've hatched from my own flock... so I'm not new to ducks I am however new to this problem! I bought 4 Khaki Campbell ducklings at the farm store last weekend (a new breed to me). They were all several days old at this point, with one being younger then the rest.

2 days later, I found the little one dead in the morning. Okay, the little one dying isn't such a surprise... but then in the middle of the night, I found a 2nd duck walking on his hocks (like the ducks hatched with low humidity). I isolated him but he died within a couple hours.

So now, a couple days later still... both remaining duckies were fine, growing great, eating/drinking eagerly, bouncing happy when I came to their cage... I left for the gym, came back a couple hours later and another was dead! Sure enough by tonight the last one was laying down, curled toes, but chirping away like nothing was wrong! He's still alive at this point, and I hand fed him, but not too hopeful... I keep having to roll him back onto his feet.

Anyone know what they could be affected/infected with? This is totally strange...