Sorry for your loss and I agree with your assessment about the wild birds. Stop feeding the wild birds, they can bring in much worse problems to your flock. I think you know that canker is spread through waterers/feeders and infected birds are carriers. I recommend copper sulfate treatment in...
You can get "Fishzole" here; same as flagyl/metronidazole. No script needed for fishzole.
http://www.jefferspet.com/fish-antibiotics/camid/LIV/cp/1441/cn/14001/