"Thanks for drawing my attention to this. Very interesting indeed, and confusing!

 

I've previously commented on the earlier pictures to both Steve White and Tony Disley. I favoured littoralis. 

However this assessment was really based on the one seemingly diagnostic feature visible, distinct green or olive tones in the rump. 

All the other features of the bird, including biometrics (all overlapping: bill of 18mm would be long for Water; wing of 97 is 1mm beyond max range of littoralis and spinoletta given in BWP, - but their measurements are taken from skins), the pattern of t5 and t6, head and underpart pattern etc., seemed to me to be ambiguous, especially for a moulting bird. 

It showed no diagnostic features of Water, i.e. pale brown rump, long wedge on tertial 5 (though what is there would be quite extreme for littoralis). 

Some littloralis look more Water-like than this and some (a few) Water, even when fully moulted, look less dramatic. The wing-bars looked poor for Water but this is probably variable and dependent on how many new, duller fringed feathers have appeared during the pre-breeding moult.

 

All of that said, the bird in the latest pictures looks very like a Water Pipit, albeit one with a rather poor supercillium (but still within the range of Water). 

The pale rump colour alone (sandy grey-brown on my monitor) is a strong support, as are the basically white underparts with just a small patch of the dark flank suffusion (hidden under the wing normally in Water) visible on the rear flanks. 

The streaking on the underparts is much sharper and more Mipit like than on any Rock. The overall impression is of a Water-like brown and white bird, quite different from any normal Rock Pipit, petrosus or littoralis.

 

I'm not aware of the full circumstances of the comings and goings of this bird but I assume it has been retrapped and confirmed as the same individual, in which case the clear green tones in the first pic may have been a photographic or processing artifact - there is no precedent for this colouration in Water Pipit. I sincerely hope they never do show it!

(To the best of my knowledge, the bird has not been re-trapped to date - WCA).

 

If the bird hasn't been re-trapped I would suggest that it should be to double check it is the same one. 

In the new pics of this bird, an impression of a moult contrast is given. If this is correct then it would have to be a first-year and obviously couldn't be the same one. 

On the greater-coverts, most feathers have neat white tips but the three innermost, perhaps especially the 2nd and 3rd, appear to be of a different generation, being fractionally longer perhaps, unusually fresh looking and show a broader pale fringe and tip, which is duller grey brown. 

The whiteness of the coverts tips increases with wear. The reason why littoralis often appears to show whiter covert tips/wingbars is because first-years replace fewer coverts (none in many 2nd brood and northern birds) in the post-juvenile moult, and compared to petrosus often show coverts which are months older and consequently more faded and white tipped.  

As adults replace all coverts during the complete post-breeding moult there should never be a moult contrast in the greater coverts on such birds. 

However, ageing these birds by moult contrasts is not easy, perhaps it is just an impression of a moult contrast being given rather than the real thing. 

It does seem too coincidental that a ringed Water Pipit could have turned up independently....

 

I wonder if there is an in-hand description of the rump and upperpart colour of the trapped bird, those obvious green tones, if real, still prevent me believing it is a Water Pipit, but perhaps they can be explained away? 

If it is the same bird and definitely is (now!) a Water Pipit, and definitely did have a green toned rump once, then the single best clue for identifying trickier birds is now useless. 

If it is a Water Pipit with green tones on the rump then it should be written up somewhere. "