Disinfect the feeder once or twice a month with one part of liquid chlorine household bleach in nine parts of warm water. If possible, immerse the feeder for two to three minutes and allow to air dry. Feed birds only high quality food. Moldy seed or bread or spoiled leftovers doesn't do them any more good than it would you. Keep rodents out of food.
Mice can carry some bird diseases.
Act early and spread the word. Don't wait until you see sick birds huddled at the feeder. And tell your friends who feed birds to take the above precautions. Check your feeder for sharp edges where birds might cut themselves. Small scratches or cuts allow bacteria and viruses to infect a bird more easily.