Sunday, 28 January 2018

Detective Work

So there have been several recent Caspian Gull happenings on the Axe. Once for Steve on 9 January, twice for me last week, and now also for Tim White on 17 January. Amazing. The question is though, how many birds have been involved? One? Two? More?

I love comparing photos to work out stuff like this, and have done so several times. One year on the Axe we had a steady procession of Iceland Gulls. Photos proved that what we thought was one very white one was actually two birds. On another occasion, careful image analysis demonstrated that just one Red-breasted Goose was involved in records from Christchurch Harbour, Ferrybridge and Exminster Marshes. My sleuthing urges have been applied to Caspian Gull pics also. Learning that our Dec 2011 bird was previously on Portland in October was a particularly satisfying one, and that Steve's 2nd-winter bird from a couple of winters ago had been at Torbay a few days prior.

Anyway, I present below my latest efforts in this department...

Left: Jan 17 (Tim White)    Right: Jan 22
In the past I have used little circles, arrows and annotation to highlight similarities. In this case I think it would all get too messy, but suffice to say there are numerous common features. Certainly enough to satisfy me anyway.

Just for the fun of it I thought I would compare open wing shots. However, in the following collage I've added a couple of previous Axe Casps to the mix. The purpose of this set of images is not so much to show that my bird and Tim's are one and the same, but to illustrate both the variability in 1st-winter Caspian Gull upperwing pattern, and the similarities - those features that help confirm identification...

Always those darker-based, paler-tipped greater coverts giving a distinctive pale wing-bar that contrasts with almost blackish secondaries. The median coverts exhibit a similar pattern, also producing a definite (if fainter) wing-bar.

I briefy considered another collage comparing my bird from last Thursday (25th) to Steve's from 9 Jan, but Steve's photos are so lacking in detail that it wouldn't be very convincing (sorry Steve!). Nevertheless I feel there's enough there to persuade me that they are most probably the same bird.

Right, let's draw some conclusions then...
  • Tim's bird from 17 Jan is my bird from 22 Jan
  • Steve's bird from 9 Jan is [probably] my bird from 25 Jan
  • I have definitely seen TWO different Axe Caspian Gulls in four days!
  • But technically I didn't find either of them...
  • The ringed Dawlish bird from 17 Jan means at least 3 Casps may still be knocking about in E Devon!
  • There may never have been a better time to 'find' a Devon Casp
Point number four highlights one reason why I have never bothered with a self-found list. Even so, learning that both my birds had previously been discovered by someone else did not for one moment detract from the massive buzz I enjoyed both times!

It's funny, although I've done so little birding in recent times, whenever I do make the effort for a while I seem to get really jammy. Now that I don't mind...

2 comments:

  1. Hi Gavin - great post as always. You've prompted me to submit a record that I shelved back in 2015. I felt I didn't have enough to rule out some hybrid influence. My flight shot blurred out the pattern on the inner primaries but your photos amply illustrate how easily the appearance of these feathers can change, depending on the quality of photos. Am planning a blog post on it and would appreciate your thoughts.
    All the best. Matt

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    1. Hi Matt, thanks for that. Re inner primaries: not only do different angles/light/photo quality seem to affect the appearance of these feathers on the same bird, in my experience there can be considerable variation between birds too. I reckon the photos above illustrate that. For example, the huge beast we had in Dec 2011 had those lovely pale lozenges on the otherwise dark inner webs of p2-p4, but I've not seen another quite like that.

      Look forward to your blog post!

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