Ok...here goes.
A ~30 foot vinyl covered 1/8" cable strung between two trees. On this cable are 8 two liter plastic bottles...caps and bottom drilled for cable to pass. This allows them to spin freely so the squirrels can get no purchase while on them and they fall to the ground...a good 20 foot drop.
At each end about 4 feet from each tree is a roundish baffle of plastic. Old compartmented snack trays. Attached to these towards the trees is an aluminum concave baffle/shield. Think of Captain America's shield only about half the size. These assemblies are anchored to the end bottle on each end. There is another tray baffle after the first bottle. The edges of the aluminum have been cut away in a triangular sawtooth pattern all the way around. Each "tooth" is about 2 inches high. The concave surface faces the tree. Hanging down from each of these shields is a 18" x 30" aluminum oven liner painted olive drab so as not to spook the birds.
On the cable between the trees and these shields is an arrangement of the triangular pieces cut from the shields...roughly 30ga aluminum. Barely visible in the picture below on the right side. They are spaced about one inch apart and are fixed to the cable so they do not move or spin.
This cable is fed through a screw eye on one end to allow the assembly to be lowered and raised after the bird feeders have been refilled. Between each bottle is a carabiner that allows attachment of the bird feeders.
Squirrels have tried to jump from the trees to the feeders but the oven liners block them in mid-flight.
They have tried to traverse the cable to get closer to the feeders but the triangular (and sharpened) aluminum "teeth" do not make for an easy or painless route for the squirrels.
I have observed their many attempts to defeat this arrangement but after about 2 weeks of trying, they have given up.
This defense was arrived at after about 2 months of trial and error. The toothed aluminum baffles and the aluminum "teeth" on the ends of the cable were the last addition. I went this route because passive defenses were failing. The aggressive part (aluminum teeth) gave them a good incentive to piss off and go try the neighbor's yard.
It looks like Rube Goldberg invented it but it is 100% effective. My wife loves her birds and hates squirrels. Happy wife, happy life. Plus the engineer in me accepted the challenge and won.
