Bluebirds have faced a serious population decline. A combination of habitat destruction, feral cats (letting fluffy roam free) and because house wrens and european starlings compete for nesting space. The house wren and european starling are well, duh, originally from Europe. Bluebirds require nesting cavities (ie, holes in trees). The European Starlings and House Wren also nest in places like holes trees. So building bluebird boxes and keeping out the house wrens (the holes you build in a box are too small for the starlings to enter) has basically kept the Bluebird from slipping into a serious survival situation. The box on my porch was one of those ones the kids paint and hang because its seemingly cute (luckily it is a decent sized box). So the bluebirds found it because they are that strapped for natural nesting places due to the infestation of the house wren and european starlings. So when I get back home I'll be building them a real bluebird box. I almost have to, since they've already moved in and had babies that basically means they have declared my front porch as home. I want then to have a spacious bluebird box with predator control built in to keep out predators.