What is occurring?

I went down to the Gateshead office for a day of the usual irritations and back again on a very crowded and noisy train. I was pottering about in the kitchen cooking some food and thinking the gulls were being particularly noisy when I looked up and saw this stand-off (beak-off). They were standing tugging at each other long enough for me to dash for the camera, although just after I'd taken this they lost their balance and had to regroup on a lower gable. Thing is, I think they're technicaly different species (lesser black-backed on the left, herring on the right?). And I'm not sure that they were fighting (there was no flapping).

Very strange creatures.