At 6.00pm I heard YTBs noisily flying past. I grabbed my camera, forgot my phone, and ran out to follow them. There was a commotion in one particular tree, where I saw at least 10 settle, that had flown in from the west. Others flew in from the east, and then more arrived and settled either in the initial tree, or in other trees close by. I assume the flock had begun amassing well before I arrived. I could hear at least four juveniles begging, and I believe I heard (and somehow videoed), one being fed - perhaps a newly fledged YTB? By 6.15pm all was settling down. Then, at 6.25, for some unknown reason, the tree erupted, and a huge number of YTBs (I would estimate 20 - 30) rose from the canopy, calling loudly, wheeling together in a dark cloud, and disappeared into the gloaming, heading up the hill. The YTBs in the surrounding trees then cried out as the main flock were leaving. I didn't see these birds fly off (there are a lot of big trees around), but within a couple of minutes, the night was silent. Overall, I would estimate a minimum of 30 - 40 YTBs had visited our little patch of the woods, settled briefly, then moved on for the night. Amazing!