So sorry! That's terrible & even worse when you can't figure out a reason or cause. I'd say get to an avian vet with poultry experience, not just a parrot vet, but I know that's easier said than done. I have no vets that can help me with poultry, so I have to scour this site and the web looking for info whenever I need help.
I've had 1 Rooster with similar symptoms, swollen head, something going on with his comb, lethargic, etc. but...know that many illnesses mimic eachother & share symptoms. In my case, not one of my other chickens or roosters have had it.
I didn't know whether it was bacterial, exposure to some chemical the previous owner used or fungal. I still don't know exactly what it was!
I treated my boy for mites, bacterial & fungal, both oral meds & topical creams. I figured these remedies cant hurt him, so might as well try. I also separated him from the others, set him up in a large cage. I added vitamins to his diet, grinding up the vitamins & rolling live mealworms in it. It took 6 months of TLC but he did recover.
At one point I thought it was over for him, he lost weight & could barely stand up, & when I took him outside he walked like he was drunk. I increased his vitamin D & moved his enclosure to allow for more sunshine, as well as shade. That winter he was in a climate controlled area, no colder than 50 degrees.
Antibiotics that address bacterial may or may not help. I was limited to what I could get...penicillin, doxycycline, metronidazole or enrofloxin. So I tried each, not all at the same time. Then probiotics after each treatment.
His comb looked really bad for a few months, I tried every remedy I could find. The best seemed to be topically applied, & I rotated these, cleaning 1st, then applying.
I don't know if this will help you, but I hope you find out what is going on. If there's a place you could take stool sample or have them get a blood sample, it may give info to lead you in the right direction. Best of luck!
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