I'm so very sorry to hear about the loss of Pixie. He was a very special one. I understand how hard it is to say goodbye to our precious ducks.
I had a crested duck who looked very much like Pixie.
Regarding the cause of death, we could speculate, but without a necropsy that's all it is. Even veterinarians often don't know without a necropsy.
My ducks have also mourned the loss of their flockmates. It is very hard. They quack loudly, calling for the missing duck, behave oddly, and seem clingy to each other and me. I try to give them extra mealworms, fresh water, take them on walks, ect, to try to make them feel better. I have lost 4 ducks now. And it has taken anywhere from 3 weeks to 4 months, depending on which duck passed away, for my remaining ducks to recover completely. But the worst grieving has usually passed with a few weeks.
I had a crested duck who looked very much like Pixie.

Regarding the cause of death, we could speculate, but without a necropsy that's all it is. Even veterinarians often don't know without a necropsy.
My ducks have also mourned the loss of their flockmates. It is very hard. They quack loudly, calling for the missing duck, behave oddly, and seem clingy to each other and me. I try to give them extra mealworms, fresh water, take them on walks, ect, to try to make them feel better. I have lost 4 ducks now. And it has taken anywhere from 3 weeks to 4 months, depending on which duck passed away, for my remaining ducks to recover completely. But the worst grieving has usually passed with a few weeks.