After considerable experimentation over the years, I have finally developed a way to squirrel-proof bird feeders that are attached to a tree trunk. I staple 12-foot long, overlapping strips of Mylar to the tree (I bought mine at a garage sale). Then I attach the feeder to the tree via four eyehooks that are screwed into the sides of the feeders. A single nail driven into the reverse side of the tree holds the upper wire and prevents it from slipping down the tree.
Even when the squirrels get into the tree and to the top of the plastic, they usually don’t land on the feeder. Since squirrels can jump about four feet from the ground, the feeders are placed at a height of five feet, leaving a span of plastic more than six feet above the feeders. I have also covered the protruding limbs with this heavy gauge plastic, a material so thick it is hard to drive a nail through, and impossible for a squirrel to pierce.