Use hot water to make a weak bleach solution (one part bleach to nine parts water). This proportion of bleach disinfects and helps loosen debris. A stronger bleach solution is not necessarily better and may damage the feeder.
Soak the feeder in the bleach solution, then use a rag or a medium-bristled brush to clean all feeder surfaces. Rinse very well with lots of water. Let the feeder air dry, in bright sunshine if possible. When fully dry, refill with fresh birdfood.