Wednesday 7 February 2018

Slaty-backed Gull: variation in saddle shade

How many shades of Grey? Slaty-backs and Vegas milling around an outfall on a calm sea. This is not unlike one of those kids puzzles where you have to find the matching pair amongst all the similar looking alternatives. Only in this case there isn't a matching pair to be found.

I took the above shot in Mie early in March 2016, it was one of the rare days at the site that the sun and wind weren't trying to out-do each as the biggest obstacle to gulling. At the time I was intent on the two Slaty-backs (second and third from the right) and their differing saddle shade. It was only back home going through the images that I noticed the Vegas are doing their level best to express their individuality in this shot too.

On more than one occasion concerns have been voiced about the paleness of some potential Slaty-backed Gulls in Europe and North America. I've struggled to see the problem in images of the birds I've seen, more often than not I see perfectly normal saddle shade for Slaty-back from my Kansai perspective.

To me surprise goes in the opposite direction and every so often I come across a really dark Slaty-backed that stands out from the crowd and I wonder if this is what observers elsewhere expect to see from a vagrant. The vast majority of birds I see aren't so much darker than the dark end of the Vega spectrum. It's very doubtful you'd notice a 'local' Slaty-backed if quickly scanning a Vega flock, especially in sunny conditions. Perhaps observers coming to the region in winter and seeing Slaty-backed against a snowy background might get the impression of a much darker gull? Personally I never see the SBG/snow combination... fortunately. Coming back to the above image, if we could switch on the sun, at a glance the paler of the two SBGs probably wouldn't appear any darker than the Vega on the left as they turn and twist on the water.

Olsen's Gulls says there's "very little" geographical variation and cites King & Carey (1999) that northern populations tend to be larger and possibly have darker upperparts. If it's true that there's little Geographical variation, then the difference I see here must be down to individual variation and in that case I'd conclude that very dark birds are scarce to rare. I wonder whether observers further north would disagree?

This is the darker of the two Slaty-backs in the opening image and it's the only strikingly dark SBG I have images of from recent years apart from the gull last weekend. Older digiscoped images are of much lower quality.

This is a slightly lightened image of the same bird. In all these images, at differing angles, the most striking feature to me is how little contrast exists between the saddle and primaries.

The following images are all from last weekend and include my first 'super-dark' SBG of the winter. It's always worth bearing in mind Vega shows a wide range of saddle shade.

This is the local SBG gold standard, and the very distinct contrast between saddle and primaries is obvious. Images of gulls always don't always convey the reality of the observers' experience, it's often so difficult to judge shades of grey from images; different angles, different light conditions, birds compared with different individuals and so on. However this saddle/primary contrast is unequivocal and I think the clearest way to indicate the difference between the lighter- and darker-saddled birds. 

And another typical SBG.

The same SBG with a Vega. This SBG my be standard in appearance but it didn't seem the brightest SBG on the beach, it repeatedly brought its catch back to the same spot where the same Vega would steal it.

The same bird on its (rather pointless) hunt for shellfish.

Another SBG in flight, here looking slightly darker only as a result of the angle.

I was quite happily getting shots of typical Slaty-backs when way along the beach I spotted a 'black-backed' gull. This had to be a dark-end SBG; the other candidates would all be first records for Japan. So off I went, as fast as the softer sand higher up the beach would allow. The light was a tad bright by the time I got there and, if anything, even better to bring out the shades of grey.

Dark-saddled Slaty-backed Gull with Vegas (2 Feb 2018). These gulls are clearly blackish-backed whereas the standard birds are... well, slaty.

Again, from a different angle there's still little contrast between saddle and primaries.

A standard SBG (sub-adult) with the dark adult to the right. That's a Taimyr Gull in the background.

The typical sub-ad SBG on the left and a Vega on the right. This was as near perfect gulling light as you could wish for.

A more heavily cropped version with just the two Slaty-backs.

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