is there any chance it was bitten/stung by some insect or snake? Please post full details and photo of what your bird looks like now, on the "emergencies, diseases, injuries, cures thread." Is there any chance another of your birds or animals may have attacked him?
For a swelling, I think maybe an ice pack could bring it down, while waiting for a response from the emergency posters. At this hour of the night, maybe not many people are still around. I'm sorry I'm no expert. If it is earlier at your time zone, could you call a vet for some advice.
IS the swelling discolored? Is it hot to the touch? Is your bird able to stand/walk/ eat/drink? Is it exhibiting any paralysis? If so I would think a toxin was injected by sting or bite. Include all the answers to these questions in your ER posting. I hope your bird can be saved.