Here is a Wild Birds Unlimited squirrel proof feeder.  These feeders work well to keep the squirrels from eating all of your seed, but they also have lots of places for seed to get stuck--then, all it takes is a little bit of rain to turn that seed into some nasty stuff.  Here are some photos of the nasty stuff inside the feeder before cleaning:

 

 

 

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Here is some of the nasty stuff that was inside the feeder

Here is some of the nasty stuff that was inside the feeder