The Observation bluebird house has one of the best designs on the market. It opens on each side and each has a clasp to prevent predators from getting in. One side has Plexiglas inside the door so you can look in and make sure the chicks are not in distress. The Plexiglas also assures that the chicks won't fledge (leave the nest) before they're ready. The other side opens for easy cleaning once the chicks have left the nest. The floor has a raised grid for drainage and stop blowflies from causing problems. 

Dimensions: 12.5" H x 6" W x 8.5"D