PHOTO GALLERY
NOTE: Because of its heavy bill, this individual was initially suspected of being a second-year Kelp Gull, but Alvaro Jaramillo pointed out that a Kelp Gull should have a larger and whiter head, smaller and darker eyes, a yellower bill, a darker back, shorter wings and whiter tertials. The observers thought its mantle was too dark for a Yellow-legged Gull and dismissed it as a Lesser Black-backed Gull--until Nick Rossiter suggested otherwise in a post to ID Frontiers.
RESPONDENT 1: Nick Rossiter <nrossiter@supanet.com> [in post to ID Frontiers on 3 May 2002]
I thought a second-winter in December shown earlier on the Trinidad site (recorded 2 December 2001) which was identified as an LBBG, looked much more like southern atlantis with its chunky bill with steep culmen angle and prominent gonydal angle. The chest is also bold and, with the usual caveats about mantle colour in photographs, the mantle shade and colour (blue-grey) also appear right for atlantis. The pale head with a pronounced eye patch is also spot-on.
The likelihood of atlantis occurring in Trinidad at some time must be fairly high because the NE trade winds flow straight from the Canaries to the West Indies.
Interestingly some recent DNA studies have differentiated southern atlantis from northern atlantis, atlantis (particularly the southern form) from michahellis and indicated that atlantis pre-dates michahellis (Liebers, Helbig and de Knijff, Molecular Ecology 10 2447-2462, 2001).
NOTE: The following comments are in response to private queries from Floyd Hayes.
RESPONDENT 1: Nick Rossiter <nrossiter@supanet.com>
I'm convinced this bird is an atlantis (Atlantic) Yellow-legged Gull. Besides the points I mentioned in the email to BIRDWG01, I would add that the tail band is broad, the back-end is not very attenuated, the tail seems to reach as far as P5/P6 (P3/P4 with LBBG), P9 and P10 seem to be the same length and the back of the head is quite a substantial solid affair. It's the bill though that is so characteristic of atlantis and so unusual for LBBG.
I note your comment on the mantle colour. I agree it looks dark but there's obviously a mixture of brown still with the emerging adult grey and I wonder whether this is not distorting the impression.
I've put together in an image file your gull and some atlantis. The others are from September and March/April but yours seems to fit in well on structure and appearance, with a bit of interpolation on moult progression.
RESPONDENT 2: Martin Reid <upupa@airmail.net>
I lean strongly towards Azorean YLGU on this bird, due to the overall structure, and especially head and bill structure which seem too extreme for LBBG and in combination with the non-LBBG-like rear-end structure, rule out LBBG, IMHO. The mantle does seem rather dark - but is it too dark? I also considered KElp, but feel the mantle os not dark enough and the eye too whitish for that taxon. Note that Azorean YLGUs have paler (often whitish) eyes than other YLGU forms. Comparing the mantle tone to the LAGUs must be done with care, as the birds appear to be softly backlit - a dangerous situation in which to attempt mantle comparisons! Just look at the different shads in the LAGUs due to their differing angles.... Attached is a scan from an article on
Azorean YLGU on BIRDING [not posted]. I don't know how the darkest Azorean birds compare to LBBG, but I've seen michahellis in with graellsii back in the UK, and Id estimate that the difference in mantle shade was no greater than that exhibited in the attached photo - thus the mantle shade of your gull is not so dark as to firmly exclude the darkest Azorean YLGUs. I have not considered a hybrid KELP x ?, as that is an exercise in futility at this point!
RESPONDENT 1: Nick Rossiter <nrossiter@supanet.com>
I remain convinced that this is atlantis. As said earlier I think the structure is perfect for atlantis and extreme for lbbg. The mantle shade does not seem too dark for atlantis. Some 2w can appear darker than they really are because of residual brown markings I suspect. I will prepare a more detailed argument over the weekend.
RESPONDENT 1: Nick Rossiter <nrossiter@supanet.com> [in response to query from Martin Reid]
Some results are shown below. It looks as if atlantis is Munsell 4.0-4.5 and graellsii 3.3-4.1 so there is a very slight overlap. Intermedius is 2.5-3.3 and the Dutch intergrades are inbetween graellsii and intermedius so I suppose they'd be around 3.0-3.5. So the difference between a Dutch integrade and atlantis would be the same as between atlantis and michahellis if scale is linear i.e. easily recognisable in appropriate lighting but readily confused otherwise.
Sources:
http://www.nrossiter.supanet.com/ylg/adultylgnotes.htm
Other researchers have also found that atlantis is darker than Mediterranean michahellis. Jonsson (1998) quotes 6.0-7.0 (mean 6.3, eastern populations highest) on the Kodak Grey Scale, where higher numbers mean darker plumage, compared to 7.0-7.5 for atlantis. Bermejo (1999) quotes 5.5-6.0 for Mediterranean michahellis, 5.25-5.50 for Iberian Atlantic michahellis and 4.25-5.0 for atlantis on the Munsell Index where lower numbers mean a darker plumage. Bermejo (pers. comm.) also considered the birds in south Morocco and western Sahara to have very similar mantle shades to those of Atlantic Iberia. Dwight (1925) rated atlantis as deep neutral grey compared to light neutral grey (two shades paler) for Larus cachinnans and dark neutral grey (one shade darker) for graellsii.
http://www.geocities.com/RainForest/Canopy/6181/gray.htm
GULL IDENTIFICATION WEBSITE Steve Hampton
gives Munsell 3.5 for graellsii
intermedius 2.7-2.8
atlantis 4.0-4.5
michahellis 5.0-5.5
master page is:
http://www.birdsnaps.com/fuscus/fuscus5cy/05cyaugbarth.htm
Birdsnaps - Gulls from the Netherlands
Mars Muusse marsmuusse@zonnet.nl, Amsterdam, the Netherlands
Theo Muusse theofiel1969@zonnet.nl, Dordrecht, the Netherlands
Bert-Jan Luijendijk bjluijendijk@planet.nl, Oud-Beijerland, the Netherlands
From Barth:
3.3-4.1 Munsell for graellsii -- most 3.5-3.8
intermedius 2.5-3.3 most 2.7-3.1
fuscus 1.9-2.8 most 2.1-2.3
RESPONDENT 1: Nick Rossiter <nrossiter@supanet.com>
Features of gull (NR):
Bill: fairly short but noticeably thick and heavy with a pronounced gonydeal angle giving a drooping appearance and a fairly steep angle at the culmen giving a blunt tip. The basal half and the tip are a pale yellow with black on the entire cutting edge and in a broad subterminal band.
Head: quite large and heavy with a squarish appearance on the rear and a sloping appearance at the front. Looks rounded in some poses. There is a dark smudge around the eye and a small dark spot behind this area but the rest of the head carries only faint steaks and is quite white in places particularly on the nape and the forehead.
Eye: already quite a pale yellow.
Neck: thickset.
Front-end: quite bold and protruding chest; upper breast has residual brown markings on the sides.
Legs: short and thick, about 2/3rds the length of the body above; pale dull yellow with a flesh tinge.
Wings at rest: fairly long but not that attenuated an appearance.
Tail to wing: no very good shot of this but it looks as if the tail comes to about half-way between P6 and P7.
Primary points at rest: look bunched with P8 close to the end and P9 and P10 equal longest.
In flight: wings look long, primary and secondary moult has been completed, tail has one feather with much less black in it -- perhaps next generation feather.
Mantle shade: at rest looks dark, perhaps same as paler forms of graellsii or darker forms of atlantis. Shade is a blue-grey. Mantle is obviously becoming adult grey rapidly with many new feathers showing through.
Comments: In the Canaries or Madeira this gull would not get a second look. It would, on structural grounds, be assigned as a second-winter atlantis. Separating atlantis from graellsii is a frequent event in these areas and I have worked out the following key for differences on which I've marked A if it favours atlantis in this case and G if it if favours graellsii. AG means no differentiation; NA indicates not applicable to this age. NI means no information.
Differentiating features between populations are that the Atlantic adult gulls have relative to: A. graellsii
1. generally heavier appearance around the head and chest; A
2. heavier bills with stubbier tips; A
3. less graceful and distinctly heavier and more ponderous appearance in
flight; AG
4. broader wings; AG
5. less attenuated appearance at rest with tail tip from midway between P6 and P7 to P7 as opposed to P6); A
6. slightly less dark mantle and bluer-grey sheen rather than neutral grey; AG
7. shriller long calls; NI
8. long calls given at 60 deg rather than 90 deg ; NI
9. blunt wingtip giving bunched primary tips on the folded wing with in lengths P10=P9, P9-P8 < P8-P7, P8-P7=P7-P6 (as opposed to P10>P9, P9-P8 = P8-P7, P8-P7=P7-P6). A
10. more white in wingtip with increased chance of white tip on P10 (45% of birds) and of mirror on P9 (23%). NA
11. less black on wingtip with reduced chance of a broad black band on P5 (46%) and much less chance of a black mark on P4 (11%); NA
12. sharper division between black and grey areas on upper wing; NA
13. not so obvious dark trailing edge on the underside of the secondaries. NA
On this basis we have 4 good structural pointers to atlantis and 3 which are perhaps harder to be objective about which do not differentiate. There is (not surprisingly) no information on the 2 calls criteria and the remaining four apply only to adults.
For second-years only I think the moult of this bird favours atlantis and most obviously southern atlantis of the Canaries, Madeira and Morocco. They will all have completed primary and secondary moult by now while some graellsii will still be finishing their moult. Also graellsii in my experience in areas such as Morocco are normally dingier in winter with more streaking on the neck and heavier head markings. Azorean atlantis also seem to have heavier head markings than some of the southern atlantis. If you
interpolate between the Madeiran bird (September) and the Morocco bird (April) on that assembly of photos sent earlier I think you get a pretty good match with the Trinidad bird in December.
So this bird does look like a good atlantis record. In particular the head and bill structure is perfect for this form. How did it get there? NE trades from the Canaries or Morocco to the West Indies seems a very good bet, just like the recent boating and sailing enthusiasts. It could also have come from the Azores -- not all the birds there are dark headed particularly by early December -- which are closer to North America.
