Standing dead trees, called snags, provide animals with food and shelter. Woodpeckers usually move in first. They feast on insects that infest the dead wood. When it's time to excavate, they prefer dead trees because they don't have to contend with sticky sap or hard, solid wood. These carpenters of the forest build new homes each year, leaving old cavities vacant for new residents. Bluebirds like old woodpecker homes. In fact, the lack of cavity trees can limit bluebird populations. Tree swallows, bats, screech owls and flying squirrels also take up residence in old woodpecker holes. Larger tree cavities house great horned owls, raccoons, squirrels and opossums. So there can be a lot of life left in an old dead tree.
http://mdc.mo.gov/forest/
Great Horned Owls are not cavity dwellers. Instead, they typically take control of open nests build by other large birds. They will also nest on contained ledges and other protected but open features of human-made structures.
Barred Owls, somewhat smaller than Great Horned Owls, are indeed cavity dwellers and will be found in trees where a large portion of an interior cross-section has rotted away, as well as in human-made purpose-built nest boxes.
Yes, dead trees are critically important.
JimmyJoeLard 3 years ago