From Julie:

Designed by Torsten Klocke, the Rohbau birdhouse "is made out of concrete with a wood door and protects its inhabitants against wind, weather, and enemies. The 35mm hole is suited for great tits, tree sparrows, redstarts, common flycatchers, and pied flycatchers." The Rohbau Birdhouse is €75 at Das Rote Paket.