Our garden seems full of robins at the moment, and they are fascinating to watch. Our feeding station falls within the territory held by one pair, but is clearly close enough to an adjacent territory for it to be worthwhile trespassing into. Since all robins look alike to us - though I suppose they themselves must be able to tell the difference, I wonder how? - I can only tell them apart by their behaviour.
The resident pair basically behave as though they owned the place, which, from their perspective, they do. Robins are not as adept at using seed feeders and the like as are the tits, siskins and goldfinches that are our commonest garden birds, but they do all right, and are prepared to dominate where they get the chance, taking up a threatening pose on the top of the pole and at times driving other birds away. This strategy does not work with the nuthatch, it may be noted.
The resident pair are quite often both there together, and they are very busy and active, which leads me to speculate that they may have chicks in the nest. I don't know where their nest is, but probably in the woodland behind our garden, I should think. I know that friends have robin nests with chicks in their gardens currently. One of the pair may stand guard while the other feeds.
The interlopers usually fly in quickly with the aim of grabbing what they can as quickly as possible. Usually it's just the one bird, sometimes the pair together. If the residents are about a fight ensues, which doesn't last for long - the gatecrashers quickly leave. This morning, one of the resident pair managed to dislodge a large chunk of fat from our fatball feeder. It fell to the ground, where the other of the pair pecked at it. Then one of the interlopers arrived, tried to stake a claim to the morsel, and was driven away by the resident birds. Sadly for them, while they were doing that a passing female blackbird seized her chance, and the morsel, making off with it!