Long seed feeders like the one by the café at Bwlch Nant y arian are good places to watch interaction between birds. I took photos last Friday of what I thought at the time were acts of aggression, but when you go through the shots, it's clear that in many cases birds pass food from one to another (allofeeding) - which itself can lead to aggression. I've put 28 photos of aggression (mainly between different species), courtship feeding, and
allofeeding between male Greenfinches on Flickr:
http://www.flickr.com/photos/78182965@N06/8465186010/in/set-72157632739288325/
Laurie Wright