"orange-billed" Terns in Europe - summer 2002

Those birders in the UK surfbirds discussion group will have seen Laurent Raty's posting on the 18th July and helpful comments to separate the various "orange-billed" Terns that have appeared in Holland and in the UK over the last couple of months. We are producing the full posting here for all surfbirders along with a full set of images that have been posted to the various galleries. Many thanks to Laurent for permission to publish this text. Join the surfbirds discussion group.

In many cases, the primary moult pattern of terns can be efficiently used as an indicator to identify individual birds, or estimate the number of different birds that are involved in a series of records. Primary moult varies individually, to an extent that it is enough to make it unlikely to find exactly the same pattern on two different birds selected at random, and assessing it can be a relatively objective process (though it is usually much easier on pics than in the field).

Below is an attempt to apply this method to the recent series of orange-billed tern records. The discussion is based only on the pics that were made available on the web. Unfortunately, these pics do not provide fully complete information about some of the birds. Hopefully birders who saw - or photographed - the concerned individuals could fill some of the gaps?

Not willing to provoke too strong headaches (as I apparently did last time I wrote something about tern moult), I'll do my best to make my points clear. There are two preliminary things that are important to know before trying to understand the primary moult of a tern:

i) When fresh, the primaries of most terns are a rather pale silvery grey. This colour is actually due to a pale "coating" or "dusting" that covers the feathers. When these feathers become older, this coating is gradually worn away; as a result of this, the outer primaries darken progressively, until they finally turn black (i.e., old primaries do not fade out paler). Only when very old, do they sometimes finally turn brownish - but this is an extremely infrequent situation on birds not in their late second or early third year. Most of the contrasts that can be seen between a tern's primaries are a consequence of this darkening: darker feathers are older, moulted a longer time ago; paler feathers are fresher, moulted more recently.

ii) Terns usually moult their inner primaries twice a year - something very counter-intuitive to many gullwatchers. Their moult into summer plumage includes a partial primary moult, that starts with the innermost pp, in late winter-early spring, and is arrested, usually at the start of breeding and after up to 6 (depending on species and individual) feathers have been replaced. The dark "wedge" that is frequently seen on the wing tip of breeding terns is a direct result of this, combined to (i): the inner hand was renewed in spring, the outer hand was not and appears therefore contrastingly darker. The moult into winter plumage is complete and involves the replacement of all primaries, starting with the innermost, as in gulls.

In most cases, I've tried to compare the contrasts that appear within the wings of each bird, rather that comparing the apparent colour of a given feather on two shots. This should normally make the conclusions relatively independent from photographic effects (though, obviously, not completely).

1. The Norfolk bird (20 June):

Orange-billed Tern, south of Happisburgh, Norfolk, UK, 20th June 2002, all copyright Robin Chittenden

The Norfolk pics are those that provide the most useful information for individual identification: the bird shown on these pics is (very) peculiar re. moult, in that its primaries appear to have been moulted asymmetrically. Look at: http://www.birdingworld.freeserve.co.uk/Stop_Press.htm

On the second and third picture, the folded right wing can easily be detailed: the tips of p10, p9 and p8 are mid-grey; then two uniformly blackish tips are clearly visible (p7 and p6); the next tip is just pointing out below the tertials, and is grey again. My first impression was that this feather was fresh (replaced in spring) but I'm not so sure anymore. When looking at the pic shown in the upper right corner of the 6th image, that show the same wing, the two blackish tips are well visible. From there, progressing inwards, there's one mid-grey tip (p5), then three distinctly paler tips (p2-4), with quite clear whitish fringes - so, it may be that only these white-fringed feathers were renewed, while p5 was old too, after all, but didn't turn black simply because of its more inner position. I can't discern the tip of p1, and get the impression that it might well have been lacking - is this correct?

The left wing can be seen on pic #4 (wings raised: far wing). The three mid-grey outermost tips can also be seen here, but they are followed by only one blackish tip (p7), itself followed directly by a very white feather.

(p6). The next feather (p5) seems greyer, more similar in tone to p8-10 (i.e., as on the right wing). From p4 inwards, the primaries appear white, just as p6. Possibly this bird lost its left p6 during winter and regrew it in spring, possibly also it just had an atypical moult scheme, I don't know. In any case, as far as I can see, this feather was renewed recently, while its equivalent on the opposite wing was not. Note that the fact that p6 is whiter than p5 on the left wing is pretty atypical, too. If these feathers had both been regrown in spring, they should be close to each other in age; p6 has a more outer position and is more exposed than p5: if anything, it should be darker. This probably adds further credit to the hypothesis that p5 is an older feather (i.e., on both wings).

I think that this pattern, if it can be assessed for other records, should allow a straightforward and almost definite individual identification of this bird.

2. The Dutch bird (9-10 June):

Orange-billed Tern, Wassenaar, Holland, 9th June, copyright Ferry Ossendorp

Orange-billed Tern, Wassenaar, Holland, 9th June, copyright Pieter Thomas

The right wing of the Dutch bird can be seen, a.o., on these two images: http://www.dutchbirding.nl/pics/pictures.php3?id=154
http://www.dutchbirding.nl/pics/pictures.php3?id=147

What these pics show is as follows: p10-8 uniformly mid-grey; p7-6 uniformly blackish; p5 mid-grey; p2-4 distinctly fringed whitish; p1 possibly lacking(?) - i.e., this is exactly the same pattern as on the Norfolk bird.

In practice, in the absence of a really strong counter-argument, this fact alone is largely enough to make me feel perfectly comfortable that they have been the very same bird. Yet, for a full confirmation, one might perhaps prefer to see (or have a description of) the Dutch bird's left wing: this wing, unfortunately, is not shown accurately on any of the pics shown on the Web.

Would there be pictures showing it? Or did anybody take notes that would include this detail?

3. The recent Dawlish Warren bird (8 July):

Orange-billed Tern, Dawlish Warren, UK, 8th July 2002, copyright Nigel Jones

Orange-billed Tern, Dawlish Warren, Devon, UK, 8th July 2002, all copyright Chris Bishop

I've not seen any other pic of this bird and none of them is really sharp.The bird is consistently facing left on the pics, so only the left folded wing can be detailed; it is probably best visible on Chris Bishop's pic. Given the quality of the shots, comments below should better be regarded as a bit tentative.

What I think I see there is: p10-8 mid-dark grey (p8 possibly marginally darker than p10-9); p7 blackish; p6 appearing whitish or very pale grey; p5 a tad darker grey than p6. With time, it is expectable that the three paler outer primaries, clearly visible on the Dutch and Norfolk pics, will progressively start to darken. P8 is a bit older than p9-10 (moult progresses outwards), so it is also expectable that it will darken a bit faster. IMO, the left wing pattern, here, is perfectly consistent with what could be expected on the Norfolk bird, all primaries being now a couple of weeks older. It's of course different from what can be seen on most of Steve Gantlett's pics, but these all show the other side of an asymmetrical bird.

Note, however, that the whiter colour of p6 as compared to p5, although obvious on all of Chris Bishop's pics (to the obvious exception of his wing-raised shot), is not visible on Nigel Jones picture. I think this is due to the wing being hold in a more vertical - less flat - position on the latter, resulting in the primaries being shadowed, so I would give more credit to the former (but this could be wrong).

The only pic that shows anything of this bird's right wing is the wing-raised "Record shot", again by Chris Bishop. The question, here, would be to know how many dark primaries compose the dark wedge that can be seen on this wing. If the bird had a normal symmetrical moult, this wedge should be restricted to the four outermost feathers (outermost pale feather: p6); if it is the Norfolk bird, it should be made of five-six feathers (outermost pale feather: p4 or p5). Although I would be more inclined to lean towards the latter possibility, I think the fuzziness of this image does not allow any firm statement.

Are there any other images, even "record shots", that would show the other side of this bird? Did any inner primary lack?

4. The former Dawlish Warren bird (18 May):

Elegant Tern, Dawlish Warren, Devon, UK, 18th May 2002, copyright Paul Marshall

This bird appears indeed to have been in full summer plumage, though I'm not at all convinced it was a full adult (just check the secondaries). There as no trace of active moult in the wing. The folded left wing appears completely, uniformly pale grey on the pics; there isn't the smallest suggestion of any significant contrast between the tips; the tips of p6-8 at least seem to have had thin whitish fringes along their inner webs.

On the right wing (based on the excellent wing-raised shot), the colour, pattern and level of wear on all of the five outer primaries appear virtually identical. Starting with this bird's right wing, to end up with the Dutch/Norfolk bird's right wing, one would need to assume that (1) three weeks would have been enough to turn two primary tips (p6-7) into blackish and (2) during these three weeks, the three outermost primary tips (p8-10) would nevertheless have remained uniform grey. I would regard (1) as highly unlikely and (2) as probably impossible. To get such a uniform and fresh-looking wing on 18 May, this bird must have moulted all of its outer primaries rather late in the winter (which would, I think, be in accordance with a subadult). This means the feathers have to be close to each other in age, each of them being separated from the next one, only by the time moult required to switch to this next feather. I would be unable to understand why, on such a wing, two median primary tips would suddenly darken to the point they turn blackish, whereas the three outermost tips would remain untouched.

Starting with this bird's left wing, to end up with the Norfolk and second Dawlish Warren birds' left wing, one would need to assume that p6 was not the same generation as p7 on the first Dawlish bird too. Under this hypothesis, p7 would be the oldest primary in the outer hand, while p6 would be the freshest one. These two feathers are exactly the same shade of grey on the first Dawlish bird, and their whitish fringes - for what I can see - appear very similar in width; if anything, the fringe on p6 is a bit thinner than on p7. I strongly doubt p7 can be significantly older than p6. IMO, this bird was not involved in any of the subsequent records.

To conclude, based on primary pattern and moult, I am perfectly happy with the Dutch and Norfolk birds having been the same individual, and the first Dawlish Warren bird having been different. Additional information about the Dutch bird's left wing could still give stronger support to this statement. I see nothing that would point to the second Dawlish Warren bird having been different from the Dutch/Norfolk individual, while I experience significant problems to make it the same as the first Dawlish individual. Additional information about this last bird's right wing might either confirm or infirm this hypothesis.

Hope this helps,

Laurent
18th July 2002