Caspian Gull...maybe

20th June 2014 - Topsham

No matter how many times I insist I'm no gull expert (I'm not just being modest, I'm really not, I prefer to think of myself as an enthusiast) I still get asked for an opinion on difficult birds. Over the years I've learned, through listening to real experts, to talk the talk about scapulars and tertials, mirrors and tongues, and I can count primaries. But when faced with a tough one, I generally rely on the Father Ted response, 'ah now, that would be an ecumenical matter'. Of course, it also pays to get as many photos as possible and show them to someone who really does know their arse from a hole in the ground.

On Friday lunchtime I got a call from Dave Stone about a claimed Caspian Gull at Bowling Green Marsh, I hopped on my bike and popped down for a look. The bird was distant across the pool and the heat haze was bad but it looked to me rather like a 2nd summer Herring Gull so I spent a while admiring the Bonaparte's Gull and looking for the apparently now resident Ross's Gull. A little later I had another look at the large gull in question and started having doubts, the bird's posture had changed and now looked a little more interesting. Soon after it flew off, nothing useful being seen due to the bright sun, and reappeared on the near shore at half the distance. I took a few photos.

Larus sp. 20/6/2014 - Topsham

Larus sp. 20/6/2014 - Topsham

Larus sp. 20/6/2014 - Topsham

Larus sp. 20/6/2014 - Topsham

Larus sp. 20/6/2014 - Topsham

Larus sp. 20/6/2014 - Topsham

Larus sp. (with Herring Gull) 20/6/2014 - Topsham

And a here's a digiscoped photo taken By David Boult (thanks) showing a little more detail on the wing.

Larus sp. 20/6/2014 - Topsham (David Boult)

News of a possible Caspian (a rarity in Devon) was put out and a few people got down in time to see it. Quite a few more will have been interested but couldn't make it so I thought I'd post some photos and give my opinion - for what that's worth. Let's hope it's seen again with better views or photos.

It's a second summer bird (third calendar year) halfway through moult to third winter, this is most clearly seen on the spread wing, P1-4 are new adult type feathers, grey with white tips, P5 is growing and shows a black band and white tip, the old P6 is missing (new P6 just starting to grow and not visible) and P7-10 are old second generation brown feathers. Unfortunately this is just about the worst age for identification purposes, there are no known diagnostic characters to look for, though there may still be clues.

First the structure: at times it did look quite convincing as a Caspian, small pear-shaped head and a long bill, long and fairly thin legs, prominent crop/breast and a suggestion of a hanging belly behind the legs. At other times it looked less good, the bill seeming to be less 'peg-like' with a stronger gonys and the primary projection really didn't look too long. It was a warm day and and this always makes gulls look sleek, small-headed and long-legged.

As for plumage there appear to be no diagnostic characters to look for, however there are a few points to make. The mantle is the same pale grey as the adjacent Herring Gull (or even the Black-headed Gulls), on a Caspian it should be slightly darker. There are a few older scapulars visible and these show hints of barring rather than the finer shaft streaks typical of Caspian. The wing coverts are mostly very faded and pale and show fine faint barring or vermiculation, the darker row of median coverts are a real problem though, these seem to be fresher than the lessers and greaters (or else why are they darker). A Caspian should show a fair number of plain grey coverts at this stage, particularly on the medians.

The bare part colours are not terribly informative at this age but I did notice a dark grey iris (I could distinguish the black pupil from the surrounding colour at least) through a scope on high magnification, a Caspian would be expected to show a rather darker iris.

So what was it? I really don't know, it certainly looked very Caspian-like at times and I don't think there's anything to rule it out but there are just to many things that don't look quite right. Maybe 'not-proven' is the only sensible verdict.

Many thanks thanks to the ever helpful and knowledgeable Martin Elliott for his input, also thanks to David Boult for sending me his photos, and to Dave Stone for the call.

As ever, I'd love to hear other views.