Yesterday I was watching a Robin which, from a distance, seemed to be head butting a planter over and over. It would perch on the dark planter and watch the green one nearby. Every so often it flew across to the green planter aiming at the same point on the rim and then fly back to the dark planter. I would guess it had spotted a spider or some other tasty morsel as many spiders, earwigs and other small creatures live under the rim.
In the end it stopped and moved on elsewhere but I don't know whether it got its meal or just gave up trying.
Something I can't say I had noticed before - that is how pale the colouring is under a Robin's wings. They are usually seen hopping about with wings closed or flying rapidly from one piece of cover to another so only the brown upper colouring shows.