Kookaburra Behaviour

I am posting this on behalf of Robert Strachan Birding Aus, I live on the Atherton Tablelands and a friend gave me this address to see if anyone out there can explain to me some unusual behaviour by our local group of kookaburras. Several times over the past few years I’ve noticed that a group of 5 or so perch on a limb of a tall milky pine tree out in the paddock. More than usually boisterous and with an enthusiasm which attracted my attention, they each take turns fluttering over to the main trunk of the tree where there is a sort of declivity below a fork of the tree. They all seemingly push their chests into the bark while flapping to keep themselves in position. They are not perched on any projection, but press themselves up against the vertical surface of the tree. There is a lot of calling and laughing back and forth, almost as if they are encouraging or challenging each other to outdo the last participant. I know that’s just me anthropomorphising, but that’s what it seems like. . . This display goes on for about 10-15 minutes and then they seem to have had enough. I would appreciate it if you could help with some explanation, as it is quite mystifying. Thanks, Robert Strachan