At a local "birding" store I found a copper plate that I could place over the too-large opening on the birdhouse. Now only chickadees can get in and out. I also had to remove the "perch" where other birds could sit and bother them. Chickadees can hold onto the opening itself, but larger birds cannot. I also purchased another birdhouse designed especially for chicadees.
Both birdhouses have been out since last fall and I was hoping that some lovely couple would move in to at least one of them.
Yesterday I noticed a pair of chickadees scoping out both houses. It was such a treat to observe. First the male checked them out, looking carefully at the outside then the inside. Much "discussion" took place and then the female followed suit. I'm not sure what their decision was. Time will tell!