Observed as follows:
LimpyChick was getting a handful of wheat, while standing, from a plate in my hand. Tooshtoosh and Co. rocked up -- but the chicks arrived fully 50 yards ahead of Tooshtoosh.
The chicks screeched to a halt. LimpyChick decided to stand her ground. Then she advanced threateningly (just a few feet). Then she bigged up. Then:
one of the chicks bigged up against her.
The whole thing lasted less than ten seconds. But there's no doubt: the chick knew that Tooshtoosh wasn't behind it, that it was on its own, and it still chose to threaten a fully grown bird.