This hollow was initiated by Kookaburras flying in and banging their strong beaks against the trunk. The excavation took more than one season, but they could presumably detect rot or white-ant activity and eventually had a breeding hollow and subsequently, in December 2006, a successful fledging. They were denied entry thereafter by a brushtail possum, who took up long term residence. The hollow has evolved and is now much larger than the initial excavation. Possums may have bred in the hollow as we have seen up to five feeding together in the vicinity.
The tree is a big (>1m diameter, 30+m high) old Angophora floribunda. The possums come and go seemingly as we don't often see or hear them in the yard. Wood ducks also show an interest in the hollow occasionally, as it is close to a dam. They did have a brood on that dam in spring 2015, so may have used the hollow that year (but discretely if so, as we didn't notice any activity till the brood was on the dam).