We have lots of small garden birds such as sparrows and blue tits in the area and I was thinking about putting up some nesting boxes. I know some people have a problem with rats and mice taking food from bird feeders, but I thought if I was a rodent, I'd think a bird box was heaven! A ready-made base of operations! I think such rodents are mostly good climbers too. They can be pretty aggressive too when they need to be. So what stops them taking over nest boxes?