Birds can enter, but the holes are too small for the squirrels. We have also heard that seed treated with hot pepper may discourage squirrels. Since birds cannot taste the pepper, they are not affected. If your feeders are suspended from a wire, place tubes around the wire. These will rotate as the squirrels try to crawl out to the birdseed and the squirrels will fall off.
Some folks revert to greasing the pole holding the feeder.
Try Vaseline or Spam. It might work and the squirrels will put on quite a show. The squirrels will win. They always do. It will just take them a while.