You need to find birds which are very used to people to start doing bird photography with a 50mm lens. These robins were in Hyde Park, so they are very used to people: even so, this shot is cropped right down to actual pixel size.
I assume this was territorial behaviour rather than courtship; the robin on the left was singing with its head bent back and its red breast puffed out, and swaying left and right, all presumably in an attempt to make itself look bigger. After a bit it chased the other bird away — or at least I think that was what was happening.