Lipomas are a possibility - a sort of fatty tumor that can be as small as a pimple or so large that when on the lower chest or abdominal area, the bird has difficulty perching and even walking.
--- These aren’t actually ‘tumors’, but can be troublesome when the bird picks at them, causing bleeding and making them prone to infection.
--- Cancerous Liposarcomas look very much like Lipomas, but are far less common. Liposarcomas are dangerous, will usually metastasize to other areas and need early, expert treatment to help insure survival.
--- Skin Cancer by the way is very uncommon in pet birds.
--- Treatment for lipomas are usually to excise them (surgical), but other options are to make a small incision and vacuum the contents out (liposuction) or to inject the growth with steroids or other choice drugs in the hope of atrophying the tissue in the hopes of it not spreading. This is called lipolysis and is the least preferred option since it doesn’t remove the growth and may contribute to complications.
--- Liposuction is also not preferred since it rarely removes all of the growth and recurrence needs to be addressed every time. In the long run, surgical removal is really the least costly and most effective.
--- The sooner these are addressed by your vet the better the outcome and much easier to control. ---