There are many bird feeders out there with the claim of being squirrel proof. There is a good deal of money being spent on squirrel proof bird feeders. There is also a lot of money spent on bird seed that is lost to these bird feeders.
There is the dome-like bird feeder that is used to try to keep squirrels away. The seed may be held in any shape or size with the top enclose with a dome-like piece. The dome does not allow the squirrel to find any purchase along the surface so that it falls off once it lands on the bird feeder. Thus, they cannot reach the seed.
Some bird feeders have a built in weight device. When the squirrel comes upon the feeder, the device is triggered. Once triggered, the seed is closed off from the squirrel not allowing it access.