An OBS member in Dublin, Ohio, Steve Habash, has been feeding his bluebirds Wild Birds Unlimited Bark Butter that he smears on the tree. 

In his words: It is bark butter on the tree.  As soon as it gets light, the bluebirds and robins are waiting to be fed.  I have no problem with you sharing.  It is hard to count the bluebirds and robins because of the constant movement.  I would say about 25-35 bluebirds and about the same number of robins.  I stand about ten feet away as that keeps the starlings away until I go inside.  All of the birds come back again 1-4 PM for a second feeding.  I have two blue bird feeders where the starlings cannot get to the inner bowl to eat the mealworms.  Nothing else has dissuaded the starlings.  The crows also eat the bark butter.  The crows throw themselves against the tree and grab at the bark butter before falling to the ground.  I have a new appreciation for the starlings and crows.  All of the woodpeckers eat the bark butter and the suet.