Sounds like a respiratory infection such as Infectious Coryza or Mycoplasma Gallisepticum (MG), also known as Chronic Respiratory Disease (CRD). Since both of these infections are very contagious and have similar symptoms, there are differences that can help determine which infection your birds may have.
Infectious Coryza normally comes with a nasty foul fishy like odor that is from the nares, face or eyes. Infectious Coryza can also come with pus build-up in one or both eyes as well as yellow thick snot that comes from the nares. Since this infection is caused by a bacteria, it is easily treatable by using antibiotics, but unfortunately birds that have had it or are exposed to it, will be lifetime carriers of the disease. During times of stress, the symptoms will resurface.
Mycoplasma Gallisepticum (MG) or
Chronic Respiratory Disease (CRD) is a bacterial infection of poultry, affecting many different species of poultry fowl, including game birds. MG is one of the
MOST COMMON respiratory disease of poultry worldwide. It is known for the consistent sneezing, gurgling or a congested chest, clear sticky snot, runny nares, coughing, bubbling/foaming or runny eyes. Since this infection is caused by a bacteria, more specifically, the
Mycobacterium bacteria, it is easily treatable but unfortunately incurable, making all birds who’ve had symptoms or are exposed, lifetime chronic carriers. During times of stress is when this infection will resurface symptoms, making birds sick again.
I would suggest that you get testing done on one of your sickest birds and send a sample off to be tested. These are the best facility labs that can help you:
RAL-
http://www.vetdna.com/test-type/avian-bird
Zoologix-
http://www.zoologix.com/avian/index.htm
Here is more information about Infectious Coryza:
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https://www.thepoultrysite.com/disease-guide/infectious-coryza
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https://poultry.extension.org/artic...oultry-diseases/infectious-coryza-in-poultry/
Here is more information about MG:
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https://www.maine.gov/dacf/ahw/animal_health/documents/backyardpoultry/Mycoplasma Factsheet.pdf
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https://www.oie.int/doc/ged/D9107.PDF
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https://afarmgirlinthemaking.com/respiratory-conditions-poultry-mycoplasma-in-chickens/
I hope the information that I’ve provided above can help you seek out an answer or help you. If you have anymore questions, feel free to ask them here or in private

. I hope all your birds get well very soon!