I've only seen it in spring and assumed it was courtship, but apparently
it's not restricted to that.
According to National Audubon, "These social birds have the amusing
habit of passing berries or even apple blossoms from one bird to the
next down a long row sitting on a branch, until one bird eats the food."
I had a flock of around 60 or 70 of these guys this afternoon suddenly
descend on my young crabapple and strip it bare of its remaining fruits
(a variety that has very small, marble-sized fruit, perfect for
berry-eating birds) and disappear again. Didn't see any berry-passing,