I've recently watched quite a few birds behaving badly.
Goldfinches can be pretty aggressive when they are on the feeders, as can the larger Greenfinch.
Siskins can certainly hold their own in a dispute, but the most aggressive I've come across, has been the Chaffinch. He'll pick a fight with anyone; and seemingly, anytime.
This selection was in much better light than the previous post.
and my favourite .......
Have a peaceful weekend, and play nice.