Canker is a protozoan infection inside the beak, and appears as yellow to grey gunk, and it is carried by pigeons. It spreads throught the water from infected birds. Pictures would be helpful. Canker is treated with Fish Zole (metronidazole) 250 mg daily for 5-6 days. Here is some reading and pictures:
http://www.poultrydvm.com/condition/canker
http://www.poultrydvm.com/condition/canker