Are there other hollows in the tree?
If a tree is old and large enough to have a hollow, there is a good chance that it will have more than one hollow. Unfortunately, some hollows you will not be able to see from the ground, and sometimes what you think is a hollow, will turn out not to be a hollow. Using binoculars (or camera zoom), scan the tree for what you suspect may be other hollows. Try not to count multiple entrances to the same hollow as a separate hollow.