Oh man - I thought canker was viral. Thank you for setting me straight!You had respiratory illness within your flock at the end of March, did that resolve completely? The vet gave you meds, what were they?
https://www.backyardchickens.com/threads/sick-chickens.1517280/
Was there a bad odor?
She has crusts around the eyes as well, so I would make an assumption based solely on the photo that she also had an eye infection?
The material inside the beak could be mucous or lesions from whatever respiratory disease you are dealing with.
Fowl Pox, which is a Virus is possible, but I think you would have noticed lesions on the outside of the beak and comb. The eye crusts don't look quite like Fowl Pox. There is no odor with Fowl Pox.
Thrush (Candidiasis) is a yeast/fungal infection, you may have detected a sour or yeasty smell, but normally there's no eye crusts that accompany Thrush unless there's secondary infection.
Canker (Trichomonosis) is caused by a protozoa, it's not a virus, but it is highly contagious.
The very best way to know what you are dealing with is to refrigerate the body and send it to your state lab for testing. Once you get more info, then you will know how to treat.
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So is pox protazoa, too? I thought pox was viral, or is that only dry pox? I don't want to misinform people.