I’m sorry about your birds. Unfortunately, it’s never a good idea to administer any medications to a bird without being prescribed by a veterinarian who has examined the birds. Amoxicillin is just one antibiotic and the birds would have to be sick with specific bacteria that can be helped with it. There are countless illnesses that can go through a bird collection and only a few specific bacteria will respond to that antibiotic. Clearly whatever is going on cannot be treated with Amoxicillin. Your only hope is to take any sick birds to an avian vet asap to have them diagnosed. You could be dealing with a virus, and antibiotics are useless against a virus. It could also be some type of airborne toxin if they are dying rapidly. If they are housed outside, it could be Avian Flu, as it is going through flocks of wild birds in some parts of the country and can be passed to captive flocks. All of these are guesses – only a Vet can determine the cause by examining the birds and running tests.