With chickens, it's always a learning experience every time something like this happens. There are many things that can go wrong with poultry. It's important that you stay current on deworming, especially in certain environments. Every day that I go out with the flock, I watch them while I am feeding breakfast to make sure that: 1) They are all active and moving around eating. No one should be quietly watching the others and not eating, or picking listlessly at food or the ground but not enthusiastically, or standing in a corner and looking unhappy, or walking very slowly for a long period. Chickens run and are active and noisy and hungry. Anyone who isn't active and noisy and hungry gets some extra observation, and if they continue to act that way for more than a few hours, they will go into a hospital pen for some extra attention and observation, as soon as I can catch them!. It reduces their stress and improves chances for survival, even though at first they won't like being in the hospital pen.
Make sure that you watch the rest of the flock closely for a few days, to make sure that this is not something contagious. You might want to consider adding some vitamins/ electrolytes to the flock's water for a few days. If you have never dewormed, do get a fecal float done on some random birds, just to see. Or, you can use a good broad spectrum dewormer, it will not hurt them even if you don't do a fecal float, although there is a withdrawal period for dewormers, it is worth it, imo.
You can get a necropsy done too if you want. But start with a fecal float if you really want to know...it's cheaper. If you get a necropsy done, it has to be done within 24 hours of the bird's death.
I'm sorry for your loss.
I’m so very sorry!
