Hello there.
This sounds very much like a disease called Mycoplasmosis, caused by a bacteria called Mycoplasma gallisepticum. It is very contagious and causes all the symptoms you are describing.
It can be controlled with an antibiotic called 'Tylan' (tylosin) injection. So get a bottle ordered from your vet. You will need to give all the birds 0.5cc and repeat it 48hrs later.
It can also affect ducks, so you should inject those as well to protect them.
Obviously hygiene is still very important, as you can spread this disease on your hands after handling the birds, so disinfect thoroughly.
Andrea
Bird Veterinarian
15 years in companion animal practise, BVM&S degree Edinburgh university
So sorry to hear that. I do hope the outcome is better for the rest.
If you get replacement chickens at a later date, it may be a good idea to give them an injection of Tylan when you get them to offer some protection again Mycoplasmosis.