Has there been any avian influenza in your area recently in wild birds? Here is a link for a necropsy if needed:
https://pdsinc.ca/Services/TestInformation/Necropsy.aspx
It sounds serious. All sorts of things can cause those symptoms, from viruses, dehydration, lead or other poisoning, or eating something bad. Make sure they are drinking enough water. Electrolytes with vitamins might help for a few days. If you lose any, I would contact the poultry lab in your...
Where are you located? Can you check your feed immediately for any mold? Is it hot where you are? In some countries viruses can cause sudden severe symptoms and illness.