When birdseed gets wet, either in a storage can or in the feeder and is allowed to sit for awhile, it will decompose or ferment, and become unsafe for birds to eat. Keep bird feeders free of spoiled, crusted seed by cleaning them periodically. Some PVC, plexi-glass or plastic feeders and suet cages can be put in the dishwasher. Let feeders dry completely before refilling them.
It is very important to clean hummingbird feeders regularly, especially those in the sun--every other day or so, if possible. You want to prevent toxic mold developing as the sugar water ferments on feeder surfaces.