The Backyard Naturalist has options for Bluebird specific feeders, like the Hanging Metal Meal Worm Feeder from Droll Yankees.

Bluebird Metal Meal Worm Feeder

Bluebirds will love this feeder. Serve meal worms, fruit or jelly!Hanging metal feeder that can hold a variety of seeds, meal worms, jelly or fruit. Bluebirds will love it!

Dimensions: 5.25″ Dia x 10″ H
Weight: .5lb
Mount: Hanging
Construction: Powder-coated metal, blue
Squirrel-resistant: No