I have several Absolute bird feeders - with a counter-weighted seed cover that can be set by weight. When a heavy bird such as a blue Jay or squirrel hops on, the seed opening slams shut. The blue jays still get food, but they have to be quick! Most give up. This gives us lots of finches, cardinals, etc.
I do miss having squirrels to practice my pellet gun skills with.
Use nothing but Safflower seed. The squirrels won't touch it but the cardinals love it.