My new project is promoting den trees... Trees with hollow centers which provide nesting and dens for wildlife. When I am on my property and notice a tree with dead wood in exposed areas, either from disease, a broken branch, a wood pecker attackor other reason, I take a chain saw and hollow out an area for the process to begin. Some of the "dens" I have made are pretty usable right away and are, in fact,being used right away...
Advantages: bio diversity and an increase in the game carrying capacity of my land. If I notice squirrel nest, I know that particular area is in need and I look for a candidate....
EDIED TO ADD:
Here's a pic I took over the weekend of some of my handy work.
If you look closely, you can see this tree had exposed dead wood. I took the chainsaw and hollowed out a space (den) the size of a football but left the opening only a few inches. I look forward to seeing what happens this year.
http://i95.photobucket.com/albums/l129/Robertesq1/IMG_0562.jpg