Tuesday, 20 February 2018

Choshi (5): Vega Gulls

Like adult Slaty-backed Gulls Larus schistisagus, adult Vega Gulls Larus vegae pose few problems at Choshi. Also, like Slaty-backed Gulls, they are everywhere and permit as good a 'grilling' as any bird you're ever likely to encounter. Not surprisingly, they showed variation in some features (e.g. wingtip pattern, shape and length of P10 tongue). However (though I made no attempt to undertake a 'comprehensive survey'), in others they were remarkably consistent (e.g. leg colour, invariably pink, with the pink varying only in its intensity). Below is a sample of typical adults.

I was also delighted to find that the colour of the orbital skin seemed quite consistent in Vega Gull (in photos at least), or at least did so in those close birds I photographed well. This was a difference from Taimyr Gull Larus taimyrensis I had noticed in birds I had been identifying as Vega Gull earlier in the winter back in Taiwan. Adult Vega Gull has rather bright orange-amber orbital skin, unlike the vinous-pinkish-red of Taimyr Gull (and the pale flesh-pink of Slaty-backed Gull). Below is a composite (with all photos taken in Choshi) showing the orbital skin of two Vega Gulls (top), a presumed Taimyr Gull (bottom left), and a Slaty-backed Gull (bottom right).

As with Slaty-backed Gulls, I spent an insufficient amount of time looking for and at 'interim' plumage stages of Vega Gull as (once the first few adult-like mantle feathers have been acquired) these were pretty straightforward to assign to category. Below are the only second- and (presumed, by black in the bill and small apical spots) fourth-winters I managed to photograph.

Also, as with Slaty-backed Gull, I left with confused as to how to identify first-winters with certainty. I was actually finding it difficult to find first-winters (or at least ones in 'easy' spots), which alone told me I must be overlooking the majority of them (as adults were abundant). My Gestalt of a first-winter was clear at the outset: It should be pale on its head (with perhaps a few streaks around the eye and no smudgy mask); it should have 'patterned' scapulars; it should have a crisp 'piano key' pattern on its greater coverts; it should have fresh tertials with notches in them; it should have a patterned base to its tail (plain tails would be 'out'); and it should be slimmer, 'fresher' overall, and longer-winged than Slaty-backed Gull, with black-looking (not brown) primaries. I found a handful of birds which fit with my Gestalt comfortably, and identified these as 'good' Vega Gulls.

However, as with Slaty-backed Gulls, there were others which seemed to show a mix of characters, and these quickly undermined what faith I had in both of my 'Gestalts'! It was becoming clear that there was overlap in the features I had earmarked as being indicative of one form or the other, and the problem was that I had no idea which should take precedence when I found both in one bird. Below is one such problem bird, which I identified initially as Vega Gull by its well-patterned scapulars, pale and patterned upperwing coverts, tertials with notches in them, and overall strikingly 'patterned' appearance. However, I switched to Slaty-backed Gull due to smudgy (brown) markings on its head and body, short thick bill, and wavy (rather than 'piano key') pattern through its greater coverts. Once it spread its tail I was immediately back to Vega Gull again, as its tail band was actually quite narrow, with a very intricately patterned base! As I look at the photos now, I'm back to Slaty-backed Gull again! So which is is? Is it a fresh-looking Slaty-backed Gull with some Vega-like characteristics? Or is it a dark and smudgy Vega Gull?

My only flying first-winter Vega Gull arguably has a similar set of problems with it, as its greater coverts become more uniform towards its outers and its scapulars are broadly dark-centred and quite plain (the same pattern as on one of the 'question mark' Slaty-backed Gulls in the previous post). However, I'm sticking with Vega Gull on account of the patterned (not solid) look to the underwing coverts, the intricately patterned tail, and the overall freshness of the entire upperside.

My conclusion to this post, then, is identical to that of the previous one: i.e. that I'm not entirely sure where the boundaries between Slaty-backed Gull and Vega Gull lie and which identification criteria should take precedence in individuals which appear to show a mix of characters. Although I was very happy with my trip to Choshi, this was the one big let down, and I could have done with at least a couple more days of head scratching available to me to have spent trying to figure this out! Above photos taken at Choshi Harbour, Chiba Prefecture 6-9/2/18.

No comments:

Post a Comment