We have bird feeders of all shapes, sizes, and types of material. Some we've had for years. We dump out leftover bird seed and shells when we refill them. If there are obvious dangers, like algae, mold, feces or feathers, we scrape those off. We spray them with a hose, but we've never taken them down and given them thorough cleanings with soap or anything else.
We recently met someone who puts her metal, mesh and plastic feeders in the dishwasher with dishwasher detergent. The other types, like wood, she washes with soap and water, sometimes a very small amount of bleach. It's her opinion that this lowers the risk of disease to the backyard birds and the other animals who eat from those feeders.
Are our birds (examples include blue jays, grackles, starlings, sparrows, swallows, thrushes, woodpeckers, American robins, crows, cardinals, mourning doves, orioles, and others) at risk from eating out of communal feeders which haven't been cleaned? If so, what diseases are most common, so we can watch out for symptoms?