A veterinarian can help you in this situation. The vet can send samples off to determine exactly why your birds are dying. A fecal floatation is also a good start to determine if your birds have intestinal parasites.
Sorry for your loss. Here are a few guesses:
Coccidiosis. This is caused by a parasite which invades the gut and causes symptoms such as blood in the faeces (although not always), and is one of the most common killers of young birds. ...
Infectious Bronchitis. ...
Mycoplasma. ...
Infectious...