Just found your Forum and wondered if anyone could knows why this happens. There are several bullfinches in our garden and we have spotted one trying to attack the window in our shed. Never seen any other birds do this - have also noted they also do this on our front window!
Welcome to the forum. This kind of behaviour is normally associated with a bird "seeing" another bird of the same species as the concept of reflection is beyond a birds brain so in effect it thinks it's another bird ie competitor for food, mating, territory etc. Great garden bird even if it is a little crazy.
Hope this helps.
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