I have (had) 4 ducks. Two years old. Two pekin, both female. Two rouin (one male one female).
Two days ago I discovered one of the pekin in the corner of the pen, unable to walk and looking very lethargic, and wanting her legs straight backwards. It had been really hot, and even though they have two small pools in the pen, thought maybe dehydrated. The male duck was trying to get frisky with the sick one, so I put her in a box with a clean bowl of water and left her overnight hoping she pulled through. Next morning she was fine with an egg in the box.
Fast forward to today. I'm cleaning the pen. She runs to corner upset, spazes, and falls over dead. Neck straight, legs straight behind her.
After doing some reading online, I thought maybe egg bound. So I dug around and squeezed as deep as I could, but didn't feel anything.
My working theory right now is she was egg bound a couple days ago, and it did internal damage coming out. All the other ducks seem fine, though the other pekin keeps walking over to wear it died and putting its head down there.