Ugh, this was the case when I worked at a nursing home. There were bird feeders placed in spots near windows, so the residents could watch the birds. The residents noticed no birds ever showed up, and when I learned that, I went out to the feeders to inspect them. Mold, mold everywhere.

I took them in, cleaned them out, sanitized them, and refilled them... but I think the birds in the area were too smart to bother with those feeders anymore. It was obvious they were neglected all the time, and I imagine the birds were well aware that the feeders weren't worth the trouble.

Sorry birds, sorry residents. I tried.