Can anyone help me with what this could be and what I can do to prevent any more dying?
Welcome to BYC!
Sorry for your trouble.
We need to identify the cause.. Are you in the US? How old? Laying yet? What is fed regularly including treats and supplements? Any recent changes.. oils spills, new feed, rat poison, new birds added.. anything.. Any lighting source used? Where were they found and at what time of day? Do you happen to still have any body that can be sent in for necropsy.. If another passes please refrigerate and send to the lab.. often free or affordable to back yard keeper in order to protect our national food supply chain.. Links if you are in the US..
State poultry labs
How to Send a Bird for a Necropsy
How many birds are still left, and their ages.. in how much space, with what type of bedding? Check and double check that fees isn't moldy. Current weather conditions? What breed, all the same? From where, and how long have you had them? All questions are just to look for helpful clues. Any photos of your set up, or flock might help.
Exact age and other factors are needed to differentiate possible causes from nutrient deficit, to apsergillosis, coccidiosis. Marek's or more.
Bacterial (botulinum, e-coli, salmonella), viral (Marek's, AI, New Castle's), genetics ( lethal genes like ear tufts, creeper), environmental (fire, flood, hurricane), parasitic (mites/lice/worms/protozoa), poison (herbicide, pesticide, rodenticide), accidental (choking/injury/predator), Toxin (deadly night shade, oleander), Nutritional (all body function including requires certain nutrients including).. This list being off the top of my head without having enough information to give any opinion on suggested course of action beyond necropsy.
Hope you get answers and the rest thrive!
