Minecraft wiki entry for Village_chaining, associated text: The process of making village boundaries overlap, with the oak village getting closer and closer to the birch village each chaining cycle, until it cannot get any closer with this method. The oak village is created first, thus the game prefers it over the birch village. Note the rounding that shifted the center to the west. This is also almost exactly the same way TangoTek chained villages in his Iron Titan.