The Gang-Gang Cockatoos (Callocephalon fimbriatum) have been hanging out there consistently, in the last few days. Look for them, as you go up a short, steep hill, just on the Bowral side of Sproule's Lane.
These birds LOVE Hawthorn berries (and other similar berries, such as "Washington Thorn", and other related plants). Let us not get hung up on the names of the plants, for they are confusing, at the best of times.
Many photos of the Gang-Gang show it chewing Hawthorn berries. It is one of their favourite foods, in season. And this is the season.
A Google Search tonight revealed a recent declaration by the National Parks and Wildlife Service that lists the Gang-Gang Cockatoo as a "vulnerable species". It makes interesting reading, and it even mentions (in clause 7) a specific threat to the population in the Bowral area, from a contagious disease to which these birds are susceptible.
I was surprised to find, when I photographed these birds today, that they were all immature males. They are developing their red heads, but have not yet properly developed the full "Mr Curly" type of crest, for which these birds are famous. Why all these immature birds together, and no adults? I didn't know.
So, I was interested to note that in the background information on the species, in the NPWS declaration (linked to above) it says: "In winter..... the species may be observed in urban areas including parks and gardens (Morcombe 1986). The species in general, and creches of young birds in particular, undertake nomadic as well as seasonal movements ...." (Clause 3).
Source: NSW Scientific Committee listing advice
So, there is my answer - I had found a Kindergarten Class of young Gang-Gangs, learning the joys of eating Hawthorn berries.
Interestingly, when Gang-Gangs are feeding intensively on berries like this, they will often allow the observer to approach very close indeed (as may be determined from my photographs). And they were sitting in these trees, right beside Kangaloon Road, with cars and trucks whizzing past, barely 3 metres away.
Gang-Gangs love their food, and when they are eating, they are remarkably calm, and placid. I love the Gang-Gang Cockatoo, but a male., with his full scarlet crest is even better than these youngsters.
Here is another link, which shows a pair, with a grey-headed female, and the scarlet crested male.