Bluebirds belong to the thrush family, Turidae, whose members are known for their singing ability and their appetite for creepy crawlies. The Turidae family is comprised of 19 species including the popular American Robin and three distinct species of bluebirds; the Eastern, Western and the Mountain Bluebird.
Most people that feed bluebirds give them mealworms. They can consume about four grams of food per day or about 12% of their body weight.
I don’t know what type of grubs you have in your compost but bluebirds might eat them. Bluebirds feed primarily on ground-dwelling insects, crickets, spiders and beetles.
When bluebirds hunt for food they sit on elevated perches until they spot an insect. They’ll then drop to the ground and capture it with their bill in a behavior known as “drop-hunting.”
Other species that also might eat grubs include: woodpeckers, titmice, nuthatches, chickadees, robins, waxwings, wrens and mockingbirds.