Wednesday, 29 March 2017

The mystery gull reappears!

I had a great day out Sunday at Dung Shr Harbour waiting around for big gulls. The forecast Saturday was for a weak cold front to pass through overnight and bring with it rain, and I fancied that these conditions might encourage one or two to enter the harbour and come within range of my camera lens. Practically the first one in was the weirdo with a thayeri-like wingtip pattern from a few weeks back, a bird I had been actively looking for ever since first laying eyes on it! Rather than disappearing on the falling tide out towards the sunset as it had done when first found, today it chose to fly first directly overhead, allowing for a much better look at that wingtip pattern!

The wing continued to look good for Thayer's Gull Larus thayeri inasmuch as it had limited black at the wingtip and most of it was pale (though admittedly greyish-whitish). A small amount of black on the inner web at the tip of p10 also seemed OK for Thayer's, as did a small black medial band distally between the white of the mirror and the grey of the tongue. Although the p10 appeared to check out, later reading revealed that it would, however, be odd for a Thayer's Gull to show so much white on the outer web of p9, and also for one to have such a large mirror there (as large as the p10 mirror on this bird). The rest of the underwing looked fine, with black tips to p10-6 showing through on the underwing in more neutral (rather than glaring) light, and nothing else. However, all the positivity really had to end there, as structurally the bird was completely out for any Thayer's!

I never really got the right kind of photos to make an assessment of structure when the bird had first shown up now almost a month ago. Add to that the fact that it had had a fish in its throat on my first set of snaps, and any accurate assessment of relative proportions was made all but impossible! However, this was clearly a very 'stocky' individual, with a long, hulking and hook-tipped bill, a bulbous head, a thick, projecting, 'swollen-looking' neck and throat, a broad 'tubular-shaped' body, and a stumpy rear end with a decidedly (bearing in mind that I've only had slim and longish-tailed taimyrensis to look at all winter) short tail. These are the structural characteristics of some kind of 'Herring' Gull, and by default they are therefore those of Vega Gull Larus vegae.

There are couple of features which may be 'out' for Vega Gull, though, but none which would seem to rule it out completely (or suggest a hybrid origin). Chief amongst these is the very pale (practically unmarked) eye (I tend to associate dark eyes with Vega Gull); second is the very bright bill (typically dull on the (few) adult Vega Gulls I find in Taiwan at this time of year); and third is the head (surprisingly free of streaks; odd, but not without precedent, with one such individual already this spring). Interestingly, the bird does seem to be a good fit for a nominate race European Herring Gull Larus argentatus, but this would seem even more unlikely than even a Thayer's Gull!

I can only conclude from the evidence of this individual that the same kind of variation that exists in wingtip patterns across the two races of European Herring Gull also exists across the different populations of Vega Gull, a conclusion which would seem to find further support in the Qi Gu individual presented in my previous post. It is therefore assured that argentatus-like individuals (an expression which seems more appropriate for these birds than does 'Thayer's-like') will pop up from time to time. Although somewhat disappointed by how this bird eventually turned out (but extremely relieved that it had at least turned up again), the first-winter Glaucous Gull Larus hyperboreus was at least once more on hand to save the day!

A productive use of time all told this winter (all of it spent flogging the gulls), even though I did unfortunately fail to add the hoped-for mega to my domestic list! Above photos taken at Dung Shr Harbour, Chiayi County 26/3/17.

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