A blog mainly about birds and birding, to supplement my website I shall add new posts on an ad hoc basis as and when I have something I think is worth sharing, whether that’s an interesting bird, something I’ve learned, perhaps about identification, or something that’s aroused my curiosity. Often there will be questions, some of which you might be able to answer... please use the comments!

Sunday, 28 September 2014

Common Tern - from east or west?

Common Tern, Burnham Overy, 20th September 2014

Well the Lesser Whitethroat may not have made the grade, but what about this Common Tern?

Since 2011 there have been a number of reports of Common Tern showing characteristics of the eastern form longipennis.  Possibly all reports in Suffolk and Norfolk relate to a single individual.  In May 2014 one was photographed at Scolt Head and in August it was seen at Cley.  I gather it had summered on Scolt Head, though I'm not clear how often it was seen there.  Knowing only of the Cley reports, and not realising it had summered at Scolt, I was not expecting to see it when I visited Burnham Overy in August.  On 25th August I picked up a tern on the far side of the channel that divides Gun Hill from Scolt Head and which, momentarily, I thought was going to be a moulting adult Black Tern, or even a Whiskered Tern.  Quickly I determined it was no marsh tern but an odd Common Tern, and then thoughts of Eastern longipennis entered my head.  It showed a fully black cap, unlike other Common Terns present, and a dark bill.  Together with the darker plumage, especially on the underparts, I wondered if this could be the Eastern Common Tern from Cley.  I'd seen photos of that bird, and I recalled the grey underparts being fairly even, but this bird looked almost patchy - or at least it was distinctly darker around the mid-rear flanks and centre of the belly, and the dark colour seemed to extend on to the underwing which I'd not remembered was the case on longipennis.  Perhaps if I'd realised the Eastern Common Tern had spent most of its time in this area and was only visiting Cley from here, I'd have reached a different conclusion, but I didn't realise that, and the odd pattern of the darker colour made me dismiss it as an odd dark-billed late-moulting Common Tern, probably contaminated to make its plumage look dark.  In any case it was a long way off and before I could get anywhere near it a dogwalker had flushed it right off.  I didn't really have an opportunity to fully resolve it, but didn't particularly suspect that I'd been watching anything rare.

News that the Eastern Common Tern was at Scolt a few days later made me think again, especially when I learned it had probably been there all along, and had just started commuting to Cley each day.  Suddenly it didn't seem unlikely that it would be visible from Burnham Overy.  Maybe my bird had been that after all?  I would never know for sure unless I saw it again, and on 18th September I did - or at least I might have done.

This time it was in the same place, asleep with Sandwich Terns and this time the only Common Tern present.  It looked dark - not just because it was with Sandwich Terns, at least I didn't think so, but I couldn't see the underparts clearly because of the angle it was facing.  I couldn't see its bill (tucked in) but from what I could see of the head it seemed to have a full clean black cap.  It was a long way off again though, and I couldn't get anything more on it.  Once again a dogwalker flushed the whole flock, but unfortunately chose to do so when I was momentarily distracted by a Redstart popping up next to me.  I didn't see it get up and fly off, and I couldn't relocate it.

No sign next day but on 20th there were 3 Common Terns there.  One of them (the only adult) looked interesting but it was sooo far off neither I nor the other birders I'd put on it could resolve it.  It looked dark (above as well as below), the bill looked black, the cap looked full and the legs looked diddly short, but it was impossible from this far off.  I headed off down the beach taking care not to flush it and eventually got a reasonably good look.  From closer I could confirm that the underparts were indeed grey, darker than I would expect on a Common Tern, but that this did not seem to extend far up to the head which I would have expected on longipennis.  Interestingly the grey did appear darkest in the places where the bird on 25th August had looked dark - I think that may well have been the same bird and the dark wasn't down to contamination!  It looked darker from above too, and on the underwing.  The bill was very nearly black, but in a good view you could see it was actually very dark blackish red.  As for the structure of the bill, I'll leave you to judge from the photos - I struggle with that sort of thing, especially as there were no adult Common Terns for comparison.  It wasn't in full summer plumage after all - the forehead was white, though I didn't think it was so extensively white as on some of the Common Terns I was seeing weeks ago.  There was some smudge of black running below the cap on to the cheek - I'd heard the Eastern Common Tern was very cleanly-marked, so was this an issue?  Structurally it looked sleek and elegant - long-winged, but was I sure it was too much for a normal Common Tern?  Not sure, no.  There was nothing to make me think it definitely wasn't the Eastern bird, but enough to make me doubt it - I certainly wasn't going to start claiming it as such until I'd done a good deal more research.

Now I've looked into things a bit more I still don't have a final answer.  Lots of questions though!

  • The bill being slightly red, though only really discernible reasonably close, is fine for longipennis, though I am less clear whether or not it matches the Norfolk longipennis. Of course western Common Tern shows a dark bill in winter, and one unresolved question for me is how (ab)normal would it be for an adult hirundo to have such a dark bill at this time of year?

  • The bill showed a tiny pale tip at close range.  I know this is normal on summer hirundo Common Terns and it seems to be present on photos of longipennis too.  But is it present on winter hirundo too?  Not visible on at least some photos of winter hirundo but I don't have much experience in winter.  If it does normally disappear as the bill blackens then does this suggest the bill has not turned and was black all along, thus pointing towards longipennis?

  • The legs looked really short both from a distance (when two longer-legged juvenile Common Terns were in the same view) and from closer.  This seems to be an indicator for longipennis.

  • In the field the grey underparts seemed to turn to white at around the lower breast, unlike photos of the Eastern Common Tern I had seen where it was grey up to the chin leaving just white cheeks.  But my photos, or at least some of them, do suggest the grey went further up, so maybe this isn't a problem after all?

  • The tone of grey on both upper and underparts is hard to assess.  In the field it seemed darker than typical Common Tern to me and to others with me, but I find it hard to be sure.  It would have been helpful if it had been among other adult Common Terns!  (It was on the first day I saw it, assuming I saw the same bird on each of three occasions, and did indeed look much darker).

  • On the first day I noticed the darkness extending on to the underwing, particularly along the trailing edge so leaving a paler centre of the wing.  I didn't realise it, but reading up since I find that's good for longipennis.

  • The forehead was clearly white - have those who have continued to see the bird at Cley (has anyone continued to see it?) noted its progress to winter plumage?  Is my bird consistent with how that bird (whether or not it's the same bird) now appears?  Similarly what about the smudge of dark extending on to the cheeks?

As this bird wasn't in full breeding plumage, the blackness of the bill isn't as clear an indicator of eastern origin as it might have been.  Maybe picking out longipennis with any degree of confidence is just too ambitious at this time of year? 

Plate 88 in the Helm guide to Terns of Europe and North America is interesting.  It shows a bird that's very similar to the one I saw, although with slightly less white on the forehead.  The caption states that it's unusual for a bird in such an early stage of moult to show a dark bill and concludes: "This and the very dark outer primaries may be characters of subadult."  If this speculation is correct then could the same be applied to my bird?  Or, given the grey plumage and short legs, could the bird in plate 88 even be longipennis?  Is that bird really within normal variation of hirundo?  If that is hirundo then I see no clear reason why my bird wasn't too.  What do you think?  Is it the wandering longipennis that I've been seeing, or just a normal western Common Tern?

Common Tern, Burnham Overy, 20th September 2014

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