Granny, I only have a "theory". If they had run out of water, but had been eating, likely they drank too much water which caused their feed to expand in the crop, this pushed fluids back up and they choked.Only think I got done was chickens and I managed to kill 5 of them and one more iffy.
I fed and watered the mama and babies yesterday and when I went out today they were all screaming bloody murder. I had forgot to put the water back in the cage. So I grabbed it and stuck it in there and they all went flying for it so I went on to do my chores. I sat down to take a break and heard mama make a weird noise. I would describe it as a short scream. I thought how odd, never heard that one before. I went to look and dead and dying babies all over the place. They had a clear liquid but very thick coming from the mouth and nose. seizures then dead. 3 were gone, watched 2 go and last one is lost balance, arching its back, on its side. Mama was fine but you could see the confusion. Broke my heart. I took her to gen.pop. and brought the last baby in. He was gasping and I tried to put him under the heat but he would just seize and gasp then go to his side. So here I sit One lonely chick in my lap.
I was sitting the dead babies on top of the cage and Binks grabbed one. I grabbed it and we played tug-o-war til I felt skin rip and I let go. I hope whatever this was dont hurt him but he was growling at me and I didnt want to get scratched up or bit so He ate 4 of them.
Or it could have been what @Wishing4Wings said.
Dehydration causes organ failure. Their bodies couldn't process so much at once, maybe.
I'm very sorry to hear that you lost them. Is Mama bird o.k.? Is the last chick doing o.k.? She may have fared better since she's older, the last little one, I would just offer water for several hours and no food just to make sure the crop empties and it's pooping. If you have Poultry Nutri-Drench, direct dose it with 1-2drops directly.
Once baby is drinking/eating, then see if Mama will take him.
Just my thoughts
