It reminds me of some "collegial" faculty meetings. 
It's a bit hard to tell but just like faculty meetings it seems like most of the aggression in the Junco flock involves a few individuals (non-scientific observation disclaimer
).
Eric - I don't think 'our' Juncos are unusually aggressive. I do offer food-grade millet because for some reason it isn't taxed. I just read a paper on the subject and these squabbles seem to be common and can be related to the wing-span of the individuals
. They are generally over in less than a second which makes the game of catching them kind of fun. If you watch enough you start to pick up the pre-squabble posturing. Come to think of it, that reminds me of faculty meetings as well. I should say that faculty meetings at our place have gotten pretty placid lately. All of the real action takes place behind closed doors with lawyers in tow. That is something I won't miss in a couple of years either.
Chuck
