Q: Does this growth mean trouble for my hickory tree?

A: The source of any mushroom, whether on trees or soil, is a fungus growing somewhere beneath. In your case, a fungus is decomposing the interior of the hickory. It has likely become hollow after many years of life. Hollow trunks are usually the result of a rotten limb stub high in the tree. The stub allows water to seep into the center of the trunk, which slowly rots away.

You can’t prevent the conks or destroy the fungus but you should consider hiring an arborist to do a tree health assessment. They can tell you whether the tree is in danger of falling. You can find tips on hiring an arborist at
How To Choose an Arborist.