Squirrels don't like 'em!
One possible solution to squirrels at your bird feeder is to offer a bird seed the squirrels won't eat. For example, try safflower seeds. Safflower is a small, white seed, high in protein and fat. Most song birds eat safflower. Squirrels and blackbirds do not.
If you want to discourage starlings, the starling's bill is made to gape open. You can observe their gaping beak when the birds forage in leaf litter or rummage in the dirt. That open gape of the mouth keeps the bird from eating the safflower.
Wild Birds Unlimited offers Golden Safflower, we call it Nutrasaff. It's actually more healthy for the song birds than is straight safflower.