David McDonald reported Pied Currawongs harassing a Ring-tailed
Possum in Canberra.
I've also noted a couple of examples of birds mobbing possums.
These related to Noisy Miners and Pied Butcherbirds mobbing a
Ring-tail Possum and Noisy Miners, Blue-faced Honeyeaters, Australian
Magpie-larks, Australian Magpies mobbing a Common Brushtail Possum.
They were written up as Woodall, P.F.  Birds mobbing possums. Sunbird 
24(1): 22-23 (1994).
As discussed in my note, the Brushtail can be a predator of birds'
eggs and thus may be a legitimate candidate for mobbing.  The Ringtail
may just suffer by association with its cousin!
Just another thought - most journals these days require statements that
the material submitted to them hasn't been published elsewhere.  Does the 
'net' constitute publication???
Peter
Dr Peter Woodall                          email = 
Dept. of Anatomical Sciences              Phone = +61 7 3365 2300
The University of Queensland              Fax   = +61 7 3365 1299
Brisbane, Qld, Australia 4072          WWW  = http://www.uq.edu.au/~anpwooda
"hamba phezulu" (= "go higher" in isiZulu)
                                                             
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