Tree cavities can provide crucial habitat for wildlife including insects.
Birds seek these cavities out in all seasons for many reasons like as a shelter, nesting place or as a food source as insects will hide in there.
If you have a living or dead tree or snag in your yard with cavities, consider letting it remain standing (if it does not pose a risk to people or buildings). For more information on the importance of snags and tree cavities in your yard and community, explore YardMap: http://content.yardmap.org/learn/habitat-types/snags/
Researchers from Ohio State and Cornell University collaborated on a project to measure the effects of increased woody habitat and rates of predation on nesting birds. (http://www.sciencedirect.com/science/article/pii/S0169204614002643). Their findings should put bird-lovers at ease who fear that increased trees, shrubs, and brush piles lead to increased predation. This was their original prediction too, but there was no significant data to support this claim. So, if you resist brush piles for this reason, reconsider, and visit YardMap to learn more.
The thicker the better as predators such as cats can’t get through.Plant bramble for example.