We recently added a squirrel proof feeder to our bird feeding station. This was a financial expedient to reduce the cost of sunflower hearts, which the squirrels love to eat in abundance. The cage around the feeder was perfect for keeping the squirrels jaws away from the feeding holes. Just the job!
It didn't take long for the squirrels to work out how to remove the lid of the feeder and dab into the tube. I am not sure that anything can be completely squirrel proof.