I finally have a squirrel-proof bird feeder - Perky Pet brand. I bought it a few weeks ago at Home Hardware and it is perfect for the little birds. As you can see, the chickadees think it is great. I have a steady stream of chickadees, nuthatches, goldfinches and juncos that feed at it, and no squirrels have been able to access the seed. The birds knock some seed on to the ground so the squirrels get a bit there as well as the birds that prefer to feed on the ground like the cardinals, red-poll finches and juncos.
The mechanism is a spring-mounted perch that will close the seed portals if something as heavy as a squirrel lands on it but will stay open when a little bird is on it. A downy woodpecker was able to feed at it the other day but I doubt that a blue jay would be successful. We were so pleased with the feeder that we bought a second one.
Now I just have to figure out a way to keep suet cakes away from the squirrels.