Which tattler is it?

Wandering Tattler seems to be a scarce and perhaps localised spring migrant in Japan, I know they there's a regular spot in Tokyo Bay and they also occur in the Izu Islands during May. I've never heard of one in Kansai but nor have I felt like heading up to Tokyo on the off-chance of getting the timing right so it's a species I have never seen here. Or anywhere else for that matter. I imagine a spring bird in breeding plumage, or even a full non-breeding bird which is less likely to be seen in Japan, would be easier to identify than a transitional autumn bird which might require a greater reliance on structure.

Quite honestly, and for no logical reason, I've always felt more likely to find a peep than a Wandering Tattler on my wader jaunts. Nevertheless last Sunday (August 31) a bird caught my attention that has suddenly put it on the radar. Image searches on the internet throw up some birds that scream Wandering, a nasal groove reaching way down the bill, a long primary projection well beyond the tail tip and a p6 falling well past the longest tertial. While others don't stand out nearly so much with the p6 tip falling around that of the longest tertial, the primary projection no greater than the longest-winged Grey-tailed and the nasal groove slightly shorter or difficult to judge in the field. According to published measurements they come very close to overlap, a degree that might only be relevant in the hand.

The bird that got me thinking at the weekend was coming along the rocky shore towards where I was sitting and it looked slightly darker than the hundreds of Grey-tailed there. I was aware the different viewing angle to all the birds on the mud flats could affect the shade of grey and that being on the rocks rather than on the mud wasn't so significant but perhaps these were still the factors that triggered the interest. I'd be hard pushed to say what it was. With the approaching bird in the scope it wasn't difficult to see the narrow supercillia didn't meet above the bill and all but disappeared behind the eye, this really peaked my interest.

Unfortunately most features were rather ambiguous. It had a long nasal groove but was it long enough? The wings reached well beyond the tail tip but were far from the length of an obvious Wandering and p6 merely reached the tip of the (worn) tertials. That could mean longish for a Grey-tailed but surely on the short side for Wandering? As far as plumage was concerned, despite the very densely marked centre breast and grey saturated sides giving it a dark appearance head-on and the supercillia there was nothing to support a Wandering identification. I couldn't see if there were any retained barred central undertail coverts from my position on the seawall and the sides of the vent were less well marked than some Grey-tailed which can have prominent chevrons the length of the undertail coverts. I dismissed it and went back to looking at other waders on the mudflats.

Two or three days later, when I had time, I had a close look at the images trying in the hope of seeing the rear tarsus detail which simply wasn't possible in the field. As the bird spent most of the time coming towards me there were few images offering even the possibility and none of the others seemed sharp enough to make anything out when sufficiently cropped. Finally, to my surprise, one of the very first and most distant images showed reasonably sharp hind legs. The rear tarsus is reticulated! So this is the tattler question: is reticulated versus scutellated really the gold standard of tattler identification? If so, this is a Wandering! And it indicates how very similar the species can look.

In retrospect though I dismissed its presence on the rocks as insignificant at the time, I'm used to seeing large numbers of roosting birds on the rocks and seawalls, Grey-tailed are always standing around inacively above the high water mark and as the tide drops they immediately head out onto the mudflats so perhaps a bird feeding on the rocks while hundreds of others were on the mud could be of more significance than I gave credit for. Another feature which came to light looking at an image of the spread wing was the white tips to the greater coverts on the upperwing are restricted to the outermost feathers (possibly due to wear?) and blackish lesser coverts on the underwing.

Below are images of the bird in question with a few Grey-tailed shots for comparison.

Coming towards me along the rocks, the supercillia don't meet above the bill and the underparts are very dark.

Very dark, heavily marked breast and short supercillium but clean vent.

A Grey-tailed (2 Sept 2012) also with a heavily marked breast but whiter ground. P6 didn't come anywhere near the tertial tip on this bird. Importantly the supercillia don't meet over the bill which rules that out as a Wandering feature. 

It's difficult to judge the length of the primaries in this shot, they're clearly longer than the tail tip but not long enough to suggest Wandering. The nasal groove is hard to assess in digital images let alone in the field but it certainly covers slightly more than the bill length.  

A typical Grey-tailed (17 August 2011) shows how long the primary projection can be but p6 is nowhere close to the tertial tips. The scutellated rear tarsus is visible here.

Grey-tailed Tattler (8 September 2009) clearly showing the ladder-like scutellated rear tarsus with "rungs" running the full width of the hind leg.

The reticulated hind tarsus of this bird looking warty or covered in yellow bubble wrap and contrasting with the scutellated front of the leg.

The underwing coverts aren't as black as I'd expected but are certainly blackish. The white tips on the greater coverts of the upperwing are only relatively prominent on the outer two feathers.