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/