Watching them here for the last couple of days, i'm amazed that they flock in such numbers. Rather than eating, which is what i'd expect them to do in these conditions, the Fieldfares seemed to have spent most of their time either fighting each other or threatening any bird that comes in range. One of them is even having a go at the Wood Pigeons when they get too close, which is great to watch. I doubt it would be quite so belligerent towards the local Sparrowhawk.
I'm not sure what the Redwings gain from flocking with the Fieldfares. They don't seem to have the same aggressive nature & being smaller lose out on the choicest feeding opportunities.
Nature is odd sometimes.