I will add some observations:
1) Predatory birds consistently outrank prey birds. A kestrel can usually run off anything but another raptor, even if the prey birds are bigger.
2) Strength in numbers. Smaller birds can gang up on a single larger bird and drive it away if they work together. One blue jay will win against a handful of sparrows, but if there are 20 sparrows, they will likely mob him and drive him away.
3) The colder it gets, the less they fight. In warmer weather, birds won't tolerate company perching at the feeder. As it gets colder, however, they concentrate on eating as fast as possible, so they fight less. First it goes to two birds on opposite ends, then three, and when the weather is deeply cold, sometimes even four.