I've discovered that this hollow is a luxury two-storey apartment for whichever possum chooses to stay, which includes a place for openly sunning themselves in the early morning. They may stay on the bottom storey for the day, or climb up into the safer (warmer?) upper storey. The presence and visibility of the possum is a point of contention for gang-gangs around the area, and on more that one occasion gang-gangs have grouped around the tree and hollow, growling and spreading their wings from above the hollow, and even (courageously) to the side of the lower storey. The size of the possum juxtaposed against the gang-gangs in real time (fifth photo), illustrates why possums are fearsome and successful predators of these 350g birds and their young. (location suppressed as gang-gangs sometimes visit this tree)