[Doves Witness]

Could be a cyst, or a tumor, or an infection in the paratympanic sinus.

Similar to another topic here:

Large growth under skin on neck below beak

Hello, Although he seemed to be managing somewhat okay, one of the 'wild' pigeons we feed on our property seemed to be having some issues with coordinated flight so we caught him in the hutch to get a closer look. An inspection revealed a large growth on his neck just below his beak. We've had...

www.pigeons.biz

 

[Doves Witness]

I'd start by checking inside the bird's beak and throat for signs of canker or yeast infection. If either of those are present, that is likely what is causing the swelling, and treatments for either are already known.

 

[Doves Witness]

Well if the swelling decreased then I'm not sure. It seems like a very good sign that it isn't the worst-possible cause (a neoplasm), and that if the cause is infection then the bird's system is fighting it effectively. In which case, perhaps just monitor the bird's condition for a few days while ensuring that he is getting sufficient vitamin A.