One of the features of a Bee Tree Hive is the 2 square inch entrance. This size opening is based on the research of Dr. Thomas Seeley.
The openings are large enough that many bees can pass through simultaneously, in both directions. However, the opening is small enough that the bees can propolise it, if they want, in order to create the exact size entrance that they want for their hive.
Above is a photo of such an entrance. The value of this for the colony is that they get to control it. It also has the advantage of creating an anti-microbial "welcome mat" that they walk across whenever they exit or enter the hive.
This is also an indication that the smaller hive cavity, and rough interior surfaces (also features of a Bee Tree Hive), have stimulated their instinct to coat the inside of the hive cavity with a propolis coating. This is huge factor in colony health. I'm always thrilled to see prolific propolis production like this.