There are reasons–important ones from those who have learned from experience of losing bluebirds to predators. Once you’ve lost them and see it firsthand, you never want to see it again. Take a look at this box and read the text below the box. This is one of my displays when I give a presentation. The VBS recently talked about this in one of their newsletters, particularly the use of the Noel hardware cloth guard over the entry hole. See diagrams below. I am happy to answer any questions. More bluebirds fledge successfully with a monitored box with predator guards. It’s been proven by the VBS statistics.
Per the VBS at their website: http://www.virginiabluebirds.org:
Predator Guard Designs
We utilize two types of predator guards to help limit predation of our bluebird nest boxes. One we call the Cat/Raccoon Guard is made of a heavy wire mesh and goes on the front of the nest box to help fend off raccoons, cats, opossums, large birds, etc. This works by backing the critters off so it is too far of a reach into the box to get the eggs or babies. The pattern for the Raccoon Guard now posted on this site is slightly different from our original version. We have changed it to make it easier to cut out and lace together. The other guard, Snake Guard, is made of round metal ducting material and is installed on the mounting pole for the nest box. This guard is primarily to inhibit access by snakes which just love to dine on little birds and eggs. This guard can also fend off climbing cats, squirrels, raccoons, etc. (It also provides a bit of a challenge for squirrels when used on pole-mounted bird feeders.)
Recommendations below from the VBS for box mounting and guards below: