Squirrels and Other Visitors
Keeping the yard's mammals out of the seed: what stops them, what merely delays them, and what they chew straight through.
Squirrels are the reason most feeder setups get rebuilt at least once. Audubon's summary of the problem is that they are very persistent and very good at two things, climbing and jumping, which is why blocking one route without the other accomplishes nothing.
The position this section takes is that the answer is geometry rather than gadgets: roughly ten feet of clearance from any launch point, and a metal baffle four to five feet up the pole. Seed choices and distraction feeding shift the odds. They do not close the door.
Articles here also cover the animals that arrive alongside the squirrels, since raccoons, chipmunks, and the larger birds that empty a feeder in a morning are variations on the same design problem.