Thursday 12 May 2016

Thayer's Gull or Vega?

I've been able to get quite a lot of birding done since early April and haven't posted on all the good birds I've seen. I'll try to catch up on a few bits and pieces as time allows, starting with this interesting gull which I initially considered an odd Vega showing some features of Thayer's. However, to my own surprise, in the course of writing this post I've swung the other way and now believe it's more likely to actually be a Thayer's.

I posted about an adult Thayer's Gull I saw in Mie on April 8 but I failed to find it when I looked again the following day. I did however find a Vega with a good thayeri-type primary pattern followed by the bird I want to write about today. Vega with thayeri-type outer primaries are uncommon but certainly not rare, and in my experience they never suggest Thayer's on closer examination. The gull I'm focusing on clearly wasn't the Thayer's I'd seen the day before therefore I was all too ready to dismiss it as the second thayeri-type Vega of the day. A second adult Thayer's the day after my first ever... preposterous notion!

Thayer's is a very scarce winter visitor further north in Japan but it's a real rarity in Kansai and I met a lot of local birders actively looking for the gull during the three or four weeks since it had first been found. I finally caught up with "it" after several attempts. I haven't seen any online images of this Thayer's in Mie and I'm wondering whether people seeing the Thayer's may have been connecting with two different birds without realising it.

Apart from the sheer improbability of seeing different individuals of such a regional rarity on consecutive days, I was inclined to think this an odd Vega because it was clearly larger and bulkier than the first gull and there didn't appear to be any difference in saddle colour between it and the Vegas present. However I only saw it for a few seconds on the beach before it flew off, there was no time to even get a record shot after first getting the scope on it. With hindsight I can easily imagine after seeing my first ever adult the day before, presumably a female which ticked all the wished for size and structure boxes, a male wouldn't look quite as convincing to my inexperienced eye. Add to that the previous day had been perfect gulling light allowing the subtly paler saddle to be noticed whereas this bird was in sunshine which could negate any slight variation. On that first occasion the gull flew further down the beach to join another flock and the Thayer's wing pattern was striking. Far more eye-catching than that of the normal thayeri-type Vega. I was really keen to get better views and hurried down the beach but failed to relocate it. It wasn't until April 20 that I managed to catch up with it again.

The first encounter; flying off (right) before I could get a shot of it at rest.

Dropping into another flock showing its "Whoa, wait a minute" wing pattern. 

I was able to get great views of the bird on the 20th, and a good set of images too. The reasons that convinced me it wasn't a Thayer's at the time, and my subsequent doubts, are as follows:-

1 The overall size was well into Vega territory, it never suggested being particularly small but as doubts creep in I suspect a male Thayer's could easily reach this size. I was worried about the head shape at the time too, sometimes rounded but distinctly angular at others. However as there will be overlap between a large male Thayer's and a female Vega this may not be as damning as I first thought. Additionally I felt the bill looked rather heavy with a quite pronounced gonydeal angle unlike the bird I'd seen recently, or as I'd expect from Thayer's, but might not a male have a more powerful bill? Looking at the images I find that the bill isn't as hefty as I'd thought, nor as large as that of the other gulls present. At the time I was struck by how uniformly yellow the bill looked compared to the Thayer's a couple of weeks earlier. Having heavily cropped some images I now see it does have the same pattern of a slightly paler base with a yellower saddle patch before the tip. Is it possible that at this later date bill colour is intensifying and therefore the Thayer's winter pattern no longer stands out?

2 The black in the outer five primaries always looked just that, it never showed the slightly grey cast I'd hope for from a potential Thayer's. Having said that flight views were fairly distant over the sea in very strong sunlight. How obvious would the difference in shade be in these conditions? The difference was clear with the previous bird but the overcast viewing conditions had been excellent.

3 The saddle at no time looked paler than Vega. It's important to stress the light was extremely harsh and the slightest change of angle could make a Slaty-backed appear the palest bird in the flock or a Black-headed as dark as a Vega. However I was paying particular attention to this, comparing it to other gulls in a range of positions and I felt did have a consistently same-as-Vega saddle. This continues to be the most convincing pro-Vega feature for me. As Thayer's averages slightly darker than smithsonianus it should be somewhere between that taxon and Vega in shade and though I hope that even in the severe light conditions I would have detected a difference perhaps it wouldn't be as easy as I imagine. However making that judgement only from the images isn't safe. It seems clearly as dark as Vega in these images I'm posting (as was my impression in the field) but I could have selected shots that contrive to make it paler.

I haven't touched on the dreaded H-word simply because I'm a bit stumped for a progenitor combination in our region that could produce this appearance. So, to the images, see what you think.

The bird in question on the right with a couple of Vega. Smaller but not dramatically so. The strong midday light renders judging true saddle colour difficult and here it looks very dark.

Somewhat on the delicate side perhaps but a female Vega could look like this too.

The Thayer's from earlier in the month showing bill colour and shape, head shape and orbital ring colour.

The bill and orbital ring colouration of this new bird is more similar here than my impression in the field had suggested, it does have a washed out base with yellower saddle over the gonys. But is the orbital ring a little too red for Thayer's?

Is the bill shape merely commensurate with a smaller female Vega, likewise the more angular but still rounded head shape? The saddle stubbornly refuses to get any paler.

At this angle the primaries have a very striking black and white pattern and look good for Thayer's other than being intensely black. Would any greyish tint be apparent in this light? The legs were deeper pink than any gulls it was directly compared to but it isn't unusual to see Vega with this dark colour - or even darker.

Such an extensively white underside to the far wing is really pushing at the bounds of possibility in Vega. This bird also has a consistently short-legged appearance. The structure isn't bad for a bulked-up version of the previous Thayer's.

The broken p5 band is also reaching the limit of the Vega comfort zone.
There's a slightly greyer tint to the primary tips on the left wing but light conditions make safe evaluation doubtful.
The dark marks in the coverts must be a photographic artifact as they don't appear in any other images. Again the outer primaries have a slightly greyish cast but I still don't trust rendition in this light.
It makes for a very odd Vega... structurally almost Common Gull-like in this shot. I'm rather surprised at how much my own opinion has swung just while writing this post, from considering it to be a Vega at the outset, to now thinking this could be the second Thayer's, rare as they are in this part of Japan, on the same beach.
Possibly the light is still playing tricks; the blackish undersides to the primaries look very bold but the light is shining through the wing.  

The only other underwing shot I managed, unfortunately this is heavily cropped to compensate as the bird was moving further out. 

The wing pattern of the Thayer's a couple of weeks earlier, as seen in optimal light conditions.

Another shot of the first Thayer's with Vega on the left and Taimyr on the right. The different angles play their part of course but in this light subtle differences in saddle colour are easier to detect.

Well, there we are. The second Thayer's of the month, a weird Vega or could it be a hybrid of some description?

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