Multiple Blackcaps

I couldn't understand why our seed feeder receives far more visits from a male Blackcap than from a female until last week, when there were two males on the feeder at the same time, on two occasions. Since then, they've avoided sharing the feeder, but after taking lots of photos and comparing the clearest ones (attached), I think I can see a recognisable difference in the feathers at the base of the beak. On one, the edge of the black cap is fairly even and smooth (hence we call him Mr Smoothie), and the other Blackcap (nicknamed Mr Roughie) has a more jagged demarcation between the black and the grey, which rises higher than the eyes. Is there an easier way to distinguish Blackcaps?

Laurie Wright