Three of these birds flew to the tree where a Gang-gang pair had been nesting previously although probably unsuccessfully (See https://canberra.naturemapr.org/Community/Sighting/4184443). One of them even flying to the hollow after a brief perch on a convenient branch. It looked around to both sides, then put its head into the hollow. I would say the bird appeared to recoil from the hollow. To me it appeared that something made the bird pull its head back out and away from the hole. For the first time also, the bird reacted to the noise of Gang-gangs flying overhead, it appeared to flinch at the noise and may be a shadow as a Gang-gang flew well over the tree. The Sulphur-crested cockatoo climbed higher past the hollow, coming to rest on the small crook in the trunk above the hollow. I thought the “recoiling” reaction of the Sulphur-crested Cockatoo, after it put its head in the hollow, made it clear the Gang-gangs had not been successful.