I noticed this behaviour with Black-headed Gulls and Coots at Sale WP... Reference my post from November last year. I see it every time I'm there.
Cheers

Watched a robin shadowing a blackbird this pm, the blackbird was rooting about in the fallen leaves and undergrowth with the robin helping itself and following the blackbird everywhere it went.

In a similar way you can watch many Gadwall shadowing diving Coot from Pengy's hide, picking up the debris that the Coot dislodges during it's dive as it floats to the surface.

Not just waterfowl, ive seen this behaviour to Redwings on Leigh harriers football pitch at one point they had hassled all the Redwings around the edges of the football pitch. Every time the Redwings moved back into the middle of the football pitch they were forced back around the edges of the pitch.

Observed similar behaviour this morning at diggle flash black headed gulls constantly shadowing 2 female GoldenEye, the Goldeneye actually took off at least 3 times and flew around the flash looking for some peace but with the flash about 70% frozen they generally came back to the same area.

I don't mean actually dabbing one with paint.
What I'm referring to is behaviour, mainly concerning Black Headed Gulls.
Its behaviour that's fascinated me for a while, and was very apparent on my recent trip to Scotsmans Flash, it's akin to a practice in soccer.
Black Headed Gulls will select either a Little Grebe, Tufted Duck, Goldeneye (especially females) or similar diving species, and they will shadow
the waterfowl relentlessly. Everytime the duck/grebe surfaces, the gull is there to try and grab their meal.
Black Headed Gulls will also try this with Great Crested Grebes and Coots, but with these they need to be careful, I've seen both species lunge at gulls
that have got too close. Coots often have a temper from hell, I've seen them snap at Black Headed Gulls, and had the rail made contact the gull would have been in real trouble.
Little Grebes though just have bear the bullying, everytime they surface the gull is there, spinning around like a clockwork toy, and trying to grab the Stickleback from their beak.
Just like in soccer, where a defender will mark and hound their opponents key striker throughout the game. Perhaps Black Headed Gulls study which Little Grebe or Tufted Duck
is having best luck fishing, and they select that bird and stay with it.
The Black Headed Gulls don't always get their own way though, Common Gulls watch their smaller cousins, and when the opportunity arises, they harass the Black Heads.
Fascinating drama, and easy to observe on the expanse of Scotsmans Flash.
Little wonder then that most of the Little Grebes on Scotsmans try and stay close to the reeds.