Is a veterinarian an option?
What you're seeing is indeed bumblefoot (pododermatitis) and is well developed in size, and will require surgery, and likely antibiotics.
If a vet is not an option, you can attempt to perform the surgery at home. The first step should include soaking the foot in either Epsom salt water or an antiseptic mixture such as chlorhexidine. That process will help eliminate any superficial contaminants on the surface and allow softening of the necrotic tissue preset on the surface so excavation can be easily achieved.
After soaking, and massaging the swelling, you can grab a scalpel and attempt to work your way around the necrotic tissue until you have the whole plug out. With most bumblefoot cases, there will be caseous material underneath that should also be excavated. Applying firm pressure to the swelling can often help squeeze any of that material out.
Afterwards, the area should be welled lavaged with an antiseptic, and packed with either an antibacterial ointment or cream or even honey. A non-adherent bandage can be placed over the wound, then apply cast padding to hold in place, and finish off with vet wrap. With such a well-developed case, antimicrobial therapy would be recommended as you may not be able to reach bacteria further into the tissue. Several antibiotics can be found here:
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Pododermatitis is most commonly brought on by keeping birds on rough, unsanitary grounds that allow degradation of the epidermal layer creating callusing, and ulceration which subsequently allows bacterial penetration into the tissue. Attempts to provide clean, soft grounds free of things the bird may get poked on can prevent another case from reoccurring.
