Why mob a Purple Martin?

I've posted at http://birdaz.com/blog a couple of lackluster photos of
Purple Martins mobbed by House Finches. This is a very common behavior here
in the southwest, but I can't figure out why they would do it: what, besides
the fairly remote potential for nest site competition, could move the
finches to such enmity?
Somebody out there must know the answer!
--
Rick Wright

Managing Director, WINGS
http://wingsbirds.com
http://birdaz.com, http://birdaz.com/blog

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