The Baltic Gulls at Appleford

Baltic Gull - field sketch

This year, I've developed a bit of an obsession with mid-summer gull watching. Right in the doldrums between the last Caspian Gulls departing and the first juvenile Yellow-legged Gulls arriving. There's only one bird on my mind here - Baltic Gull.

The nominate form of Lesser Black-backed Gull, Larus fuscus fuscus, breeds on the fringes of the Baltic Sea, northern Scandinavia and northwest Russia. It is really quite a different beast to the British graellsii and Western Europe intermedius. It takes only three years for it to reach maturity, as opposed to four years in most other large gulls. It is also a very long-distance migrant, travelling from the breeding colonies to winter in Africa, some 6-7000km south. This is probably the selective pressure that has driven adaptations such as accelerated moult and long wings in comparison to the other two subspecies of Lesser Black-backed Gull. Baltic Gulls usually migrate through Eastern Europe and hence their status in Western Europe is unclear, although it is likely that they are not particularly common as their migration does not follow the Western coast.

There seems to be a particular apathy amongst birders towards the occurrence of Baltic Gull in Britain - it's tarred by the "subspecies only" brush, and candidate small, elongated, dark-mantled Lesser Black-backed Gulls are recorded fairly regularly. For me, however, simply stringing an intermedius isn't enough - I need to be certain. Since the publication of Lars Jonsson's original paper in Birding World, it has been found that the variation in the moult of graellsii and intermedius is far greater than initially thought. This has led rarities committees to consider second summers and adults essentially impossible to separate from intermedius - so currently, there are only two types of Baltic Gull sighting that are acceptable from a vagrant perspective: ringed adults from "pure" colonies, and first-summer birds from May-August. Because of these rather strict criteria, there have only been 12 records of Baltic Gull accepted by the BBRC up until the end of 2019 - although it is certainly an annual migrant rather than the mega vagrant that these numbers would suggest.

I don't actually agree with this assessment myself and I think a combination of moult and jizz in these older age classes would be extremely supportive of a positive Baltic Gull identification, especially in small female fuscus where the structure is really quite far removed from intermedius - but it is what it is, and therefore in my quest to find an acceptable fuscus I would have to focus on ringed adults and first-summers. And let's face it - field-identifiable first-summers are much more interesting...

So, what to look for in a first-summer Baltic Gull?

I have summarised the differences between Baltic Gull L.f. fuscus and Lesser Black-Backed Gull L.f. graellsii/intermedius in the plates below. I produced these illustrations digitally using Adobe Photoshop. Please click the plate and "open in a new tab" to view full size!

Plate 1

Structure

Baltic Gull averages slightly smaller than Western Lesser Black-backed Gull, although this is often not immediately apparent in the field due to the huge variability in size demonstrated by graellsii/intermedius. What is usually obvious is the very long primary projection and attenuated rear end, which is accentuated by the differences in moult described later. Often appears quite short-legged and small-headed, but again variable.

Bill

First-summer fuscus typically have a much more mature-looking bill than contemporary graellsii/intermedius, with most of the bill base being pale yellow/pink with some residual black markings at the tip. Some individuals even have a hint of red gonys beginning to develop. In contrast, first-summer graellsii/intermedius tend to have a dark bill, although some will have the same pale base to the bill that fuscus shows. Therefore, I would say that birds which have an all-dark bill are unlikely to be fuscus, but birds with a pale base to the bill are not necessarily fuscus. Bill structure seems variable enough as to overlap completely with graellsii/intermedius.

Plumage

Baltic Gulls tend to have a very distinctive mantle colour, being a rich dark brown colour. This is also noticeable on adults, where the brown wash across the mantle immediately serves to distinguish fuscus from similarly dark-mantled intermedius, which usually have a more slaty tone.

The second-generation scapular feathers are plain brown, with pale edges when worn, producing a distinctive scaly look. There will sometimes be a dark line following the shaft of the feather, but no perpendicular anchor marks. Second-generation feathers in graellsii/intermedius are very variable but many have a Herring Gull-like pattern of anchors on dark grey feathers which should immediately rule out fuscus. Throughout summer, Baltic gulls will replace their scapulars with new third generation feathers which are very dark black-brown, almost as dark as the black mantle of adults. This should be noticeably darker than any feathers present in the mantle of graellsii/intermedius.

The coverts are plain brown with slightly paler worn fringes in old second-generation feathers, with new black-brown third-generation feathers beginning to poke through in late summer. Any kind of strong barring on the greater coverts seems to be a bad sign for a candidate fuscus. The tertials are dark centred with broad white tips, lacking any kind of subterminal barring. Birds with fuscus-type moult displaying covert/tertial barring are likely to be hybrids with intermedius.

Body/underpart feathers are variable in fuscus, but average whiter than graellsii/intermedius of the same age. The head feathers are white with with streaking around the eye and on the nape. In comparison, many graellsii/intermedius will be quite heavily streaked in their first-summer. A white underwing has been mooted as a good identifying feature for fuscus, especially when juvenile, but from looking at a large number of photos I thunk this feature is as variable as any other large gull. The tail pattern ranges from a band with barring leading up the rump, to almost completely dark retrices. The rump feathers tend to be white, rather than barred.

Moult

It is in their moult strategy that Baltic Gull deviates most from graellsii/intermedius. Due to being a three-year gull, with accelerated moult strategy resulting from long-distance migration, the extent of moult can be used to safely distinguish a large proportion of fuscus in a western European context. Before departing their wintering grounds in Africa, first-winter fuscus will replace at least eight, and usually all ten primaries, as well as all their secondaries and tail feathers. This complete post-juvenile moult is usually finished by April. In comparison, graellsii and intermedius tend to start their primary moult in May, and therefore will be undergoing this moult throughout the whole of summer. Because of this, those few fuscus that end up in Western Europe in May and July will have primaries that look distinctly blacker and fresher than all other Lesser Black-backed gulls in the area. The second-generation primaries and secondaries differ slightly from the juvenile feathers, with the secondaries having broad white tips, the inner primaries having small white fringes to the tips and with P10 sometimes having a small white mirror.

The pattern of moult in the first-summer Baltic Gull illustrated in Plate 1 is typical of many individuals in early summer (May-July), with all primaries replaced with fresh second-generation feathers on the wintering grounds. It is these individuals which are identifiable as Baltic gulls, as some slower-moulting birds will have arrested their moult halfway through replacing their primaries and would therefore not be separable from graellsii/intermedius. Altenburg (2011) states that any bird with more than eight new primaries should be a Baltic Gull - this is illustrated in Plate 2 with an individual that has retained a worn, juvenile P10. In early summer, Baltic Gulls will have no wing moult, which immediately distinguishes them from graellsii/intermedius. 

Plate 2

As summer progresses, graellsii and intermedius will start their primary moult, which invites confusion with fuscus. This is especially the case in late summer when fuscus will start its third wave of primary moult, replacing their inner primaries with fresh third-generation feathers with broad white tips. In some rare cases, they will still have retained their old P9/P10 from arrested moult prior to migration, hence showing three generations of feathers in the wing - a moult pattern known as Staffelmauser (Plate 3). Whilst this places them almost a year ahead of their Western counterparts, both forms will be in active moult and it can be challenging to assess the exact age of the primaries at distance. In flight, the juvenile primaries of graellsii/intermedius will typically look pale brown and heavily abraded, whilst the second-generation primaries of fuscus will still appear pretty black, just slightly more worn than the fresh third-generation primaries poking through. The broad white tips to the third-generation primaries of fuscus should also be obvious given good photographs. July is typically the month in which graellsii/intermedius shows an incomplete tail - once they have replaced all retrices this can no longer be used as a feature to separate them from fuscus.

Plate 3

The difficulty in assessing primary moult is especially apparent when birds are standing with a closed wing - since some intermedius will have replaced a lot of their primaries by mid-August (in some cases up to P8) simply looking for birds with fresh black primaries in late summer is futile. However, even on the closed wing, there are some clues that can be used. Baltic Gulls will still have a much longer primary projection as they will have a full set of outer primaries. In contrast, these feathers in graellsii/intermedius will still be in active moult, regrowing P9-10, and therefore these birds will look pretty truncated at the rear despite the primaries being fresh and black. This is illustrated in Plate 4 - fuscus will have about six primary tips visible beyond the tertials (P5-10) whilst graellsii/intermedius will have at most four, with the outer primary being P8 and P9/P10 regrowing behind it. Comparison with the other plumage features described above means that fuscus can still be identified with care through late summer.

Plate 4

Getting into September, graellsii/intermedius will have completed their primary moult and fuscus will have suspended its moult halfway through renewing its primaries to third-generation. At this point it becomes extremely difficult to assess primary moult and the window for identifying first-summer fuscus in the field closes.

So to summarise, the key features for safely identifying first-summer Baltic Gull in a Western European context in May-August are:

1. At least eight first-generation (juvenile) primaries replaced with second-generation, or in late summer, third-generation feathers. If P9-10 are retained first-generation feathers, this will be from arrested moult.
2. Full set of second-generation secondaries.
3. Full set of second-generation retrices (tail feathers).
4. Dark black-brown third-generation scapulars and coverts in the mantle.
5. Lack of strong barring in the coverts and tertials, with most feathers being plain brown.

Other indicative features include:

1. Pale-based bill with black markings at tip.
2. Elongated structure.
3. Whitish head and underparts.

Armed with this knowledge I began searching. After a few weeks of visiting the pit, and staring at hundreds of photos, the "moment" finally happened on 1st July. Surprisingly, it wasn't the primaries which initially drew my attention to the bird, as they were hidden behind another gull. Instead, it was the rich, brown, mahogany-toned mantle, with plain scapulars and coverts and the odd darker feather poking through. The head was white and the beak was pale yellow with a black tip. After about half a minute of watching this bird it flicked its wings slightly and for a split second its primaries were visible - tar black triangles that cut straight through the heat haze and hayfever. It really was one of those "you'll know it if you see it" moments - I had doubted my ability to actually pick this out at distance, and had been umming and ahing over the dubiously darker primaries of a couple of candidate birds in previous weeks.

In slight disbelief I waited a minute for it to do it again before phoning Ian Lewington, who luckily lives only a few minutes away and regularly watches the gulls on the landfill and gravel pit. Unfortunately it was at this point that the bird decided to go to sleep and so when Ian arrived it was displaying precisely no diagnostic features. It was also incredibly distant in amongst the furthest group of gulls some 300m away on the opposite side of the pit, and I was beginning to doubt what I had seen. A few agonising minutes passed before eventually, it woke up again and stretched its wings high above its head, revealing a full set of fresh primaries and secondaries with no obvious moult break - sealing the deal for me. Ian luckily managed to capture this on video, which was immensely helpful as this was, of course, the one day that I neglected to bring my phone scope adapter, and was really struggling to get good images just by holding my phone to the eyepiece.

After putting the news out and appreciating the bird for a bit longer, Ian suggested that we walk round to the bend in the road where it might be slightly closer. The vegetation had grown up quite high since my last visit and I struggled to peer over it - Ian, being taller and with a straight-through scope, had no such issues and quickly picked out the bird preening on the spit. After 10 minutes it took off and began flying around the pit, revealing a fully replaced tail with a diffuse band matching the intense black colour of the primaries, and with fresh white tips to each feather. It then steadily flew high north along with a few other gulls that were beginning to depart the pit, perhaps heading to Radley Lakes to roost. 

I saw this bird again on 13th July, where it showed reasonably well at a similar distance. Note how small-headed the fuscus looks in comparison to the graellsii/intermedius behind it. Interestingly, stills from the video footage of it taking off show a slight gap at the base of the primaries.

Then, on 6th August, I saw this individual a third time, and it had clearly started its third wave of primary moult, replacing P1-2 with fresh third-generation feathers with larger white tips. P3 was missing and the second-generation P4-10 still looked black and fresh enough for this feature to still be used to pick out the Baltic Gull despite it being in primary moult. This accelerated primary moult fits perfectly with the identification as fuscus - now, it is almost a year ahead of its graellsii/intermedius counterparts.

It transpires that Roger Wyatt had seen this individual on the Spit Pit (a pool near Didcot Landfill south of the main gravel pit) in the morning and had sent Ian a range of superb images. Many thanks to Roger for allowing me to reproduce them here - an upgrade on my mediocre digiscoping for sure! These photos also reveal that the bird is currently undergoing extensive covert moult, having lost a large number of greater coverts on the right wing. Note the difference in tip pattern between the third- and second-generation primaries.

Since then, this particular individual has become regular on the pit in the evening and several local birders have managed catch up with it. At the time of writing it was last seen on 13th August, so it has remained in the area for 43 days.

On 28th July I found a second individual. I had panned over a sleeping gull a few times, and noted that it had very black scapulars. Being partially obscured and with its head tucked in, I wasn't sure whether it was simply a small second-summer Great Black-backed Gull. After a while it woke up and revealed a slim, yellow/pinkish-based bill, and a really tatty, moth-eaten white head - clearly in moult, and recalling Jonsson's illustrations in his 1998 paper. The visible primaries were tar-black - success! 

This individual was far more worn, having tatty white edges to the scapulars and coverts. I find this "scaly" look really distinctive, with the combination of relatively plain, dark-brown/black centred feathers with pale worn edges looking completely unlike other Lesser Black-backed Gulls. Its structure was much less extreme than the previous individual. Upon examination of the video footage of it flapping its wings, it was clear that P10 was an old, juvenile feather, being pale brown with an abraded tip. The tail was fully replaced, and had a much thinner black band than Bird 1. 

On 7th August Ian found a third bird - sadly, I was out of county that day and therefore did not see the bird myself, but luckily the bird showed very well and Ian was able to take a series of photos which I have reproduced here. It is in a similar state of moult to Bird 1. However, it is slightly more advanced in its scapular moult and its bill pattern is different, with a hint of red gonys beginning to come through.

I saw the bird myself a couple of days later and amazingly it was on the pit at the same time as the "original" Baltic gull!

As if three Baltic gulls wasn't enough, Ian sent me a photo that Roger Wyatt had taken of a first-summer Lesser Black-backed gull on the Spit Pit a couple of weeks ago. It looks like the perfect candidate fuscus, and clearly different to the previous three birds on the basis of bird markings, and moulted covert and scapular pattern. Sadly, this is the only photo, and I'm not sure if it's possible to clinch the age of the primaries from it. To me, they do look jet-black, and the tip of P7 on the left wing (visible above where the right wing tip bisects the left wing primaries) looks rounded. P10 looks a bit ragged on both wings. I would have liked a flight shot and views of the tail... but I am sure it is one!

So probably four different first-summer Baltic Gulls at Appleford in the space of a month - really quite unprecedented when I set out searching in at the beginning of summer. These also become the first confirmed Oxfordshire records after a few potential adults and a juvenile in the past. I have no idea if they're actually as regular as this suggests - some far better gull-watchers than me have spent several years searching before finding one. I would be interested to see if this pattern is repeated next year, or whether this is a one-off mini "influx" -  I suspect the latter is unlikely as it's not been correlated with other birds reported from other sites in the UK. Really rewarding either way and good to try and clarify the status of Baltic Gull in the UK. Maybe in a few years, we'll be finding them as regularly as Caspian Gulls...

Eyestrain at Farmoor

For my Oxfordshire birding I have somehow ended up adopting three patches - Port Meadow (the original and the best), Appleford (because of the gulls and the tip) and Farmoor Reservoir. Why I visit the latter so regularly is a mystery to me as I despise birding there - it is a featureless double concrete basin and on a bad day there is very little to even look at. I guess it's close to where I live and very convenient to get to from my office, so I end up popping in a few times a week. What draws me there is the potential for rarity finding, particularly at the roost, where the numbers of gulls eclipse anything seen at Port Meadow - probably at least ten thousand in the depths of winter.

The downside to this is the size of the reservoirs and the fact that the gulls prefer to roost right in the middle of F2, the larger basin, meaning that they are typically several hundred metres distant. To give an idea of just how far away many of the birds are, the video below starts at the maximum magnification of my scope (60x) before using the digital zoom on my phone camera to produce an acceptable record shot of a first winter Caspian gull. 

The distance means that even at 60x the birds are still tiny through the scope, so features like the exact pattern of the scapulars and coverts are very difficult to discern. Combined with the fading light it's an absolute recipe for eyestrain when trying to examine a candidate bird. Therefore at Farmoor, identification of Caspian gulls tends to be based on bulk plumage features (like white head, brown greater covert bar and plain tertials). This means that some of the more borderline individuals remain unidentified, or equally that some minor Herring gull features that might indicate hybrid origin are missed. Nevertheless, it is usually possible to identify most birds in the roost and despite the distant views the sheer number of birds makes for very entertaining roost watching.

Typical views of the gulls on F2.

Since September last year, I've had Caspian gulls regularly in the Farmoor roost - most of these birds are individuals that I've had better views of at other sites in the county, which makes identifying them much easier. I've posted some of these photos before, so here are a selection of birds that I've seen recently, together with some better images of these individuals from other sites!

1w in roost on F2.

Better views at Appleford earlier in the year, although still pretty distant. This is such an amazing bird, the best of the winter by far... that extreme scapular pattern, with just a single dark line following the rachis of each feather, is my absolute favourite plumage type in cachinnans. Interestingly, this bird was dark, with a rather streaky head and dusky underwing.

Massive 1w male on pontoon on F1.

This image was taken by Mick Cunningham, who found this distinctive bird during the day at Standlake. The heavy bill, indicative of a male Caspian gull, is evident here - this is also an extremely large bird. It has been seen at various sites in the county since October.

2w in roost on F2.

Showing P10 mirrors.

Same 2w in the field at Appleford.

This rather indistinctive 1w has been seen regularly at the Farmoor roost and also at Port Meadow - this superb image was taken by Joe Wynn as he chanced on it resting close on a pontoon on F1. Below is a video of this bird in flight over F2, on an evening where it was so windy that the only option was "seawatching" and identifying the birds as they flew into and around the reservoirs! All the relevant features for identifying cachinnans in flight are visible - note the clean uppertail with neat black tail band, white underwing, contrasty upper wing and the obvious neck shawl behind the white head.

A couple of Caspian x Herring gull hybrids were also noted.

This bird was regular in the roost in February and combined the clean coverts and tertials of Caspian gull with a Herring-like head and bill. Perhaps unsurprisingly it bore a yellow ring.

The putative 2w Viking gull deserted the Port Meadow roost at the beginning of March and moved to Farmoor, where it took a strong liking to one of the blue buoys and roosted on top of it each night. Its bulk meant that it was quite dominant over the other gulls and it had no problem removing an incumbent gull from the buoy and then defending its post for the rest of the evening. In the light at the Farmoor roost the uniform beige colouration of the underparts (actually darker than the pale grey mantle feathers) was evident. All equivocal I know as these fine vermiculated plumage features can be shown by pale 2w Herrings. But to have all of them in one bird combined with a Glauc-y structure makes me suspicious and having studied it at length I think there's a pretty strong case for a hybrid here.

Yellow-legged gulls are regular at Farmoor with double-figure counts on some nights, particularly in late summer/early autumn. Some images of 1w birds below.

Small gulls, mainly black-headed gulls make up the bulk of the Farmoor roost and searching through thousands of identical individuals each night can be rather tiresome. Nonetheless there are rewards to be had - these two 1w kittiwakes appeared on F1 after a south westerly blow on 28th October.

Little gulls are regular in spring but much rarer in the winter roost so I was pleased to find these two 1w birds on 8th November.

The main passage of Mediterranean gulls occurs in March with smaller numbers in October and throughout this season I recorded a large number of individuals of all ages. In March, it was rare that I visited the roost without seeing a Mediterranean gull - the status of this species really has changed in the county, with a couple of local breeding pairs.

Numbers peaked at four on 6th March, which included two birds that were obviously paired and engaging in courtship behaviour. This consisted of chasing each other around calling and swimming in tight circles looking intently at each other. Strange beasts. These inseparable birds visited continuously for a couple of weeks and throughout their stay the thinner billed (presumably female) bird acquired a full black hood. 

I've noticed that early passage seems to consist exclusively of adults, with 1w birds only appearing in late March once the adults have petered out.

As well as the gulls, there have been a couple of other surprises - this cattle egret which few north as I was doing the roost on 2nd October was the first record for the reservoirs:

Whilst a ringed common crane from the reintroduction project on 12th March was another a good flyover and a patch tick.

In general, though, good finds have been few and far between... Hard to believe there wasn't a single white winger recorded in dozens of visits to the roost! I hope to rectify this dearth of finds during spring passage. Bonaparte's season is almost upon us and with a 1w bird moving east from Cardiff to Gloucestershire I'm hopeful that it'll end up at Farmoor in the next few weeks!

Hybrids and more

Despite the cold weather conditions providing interesting bird movements across Britain, birding on Port Meadow has been relatively uneventful. It has been relatively static in terms of Caspian gulls in the roost. I suspect I have almost reached saturation point with this species in Oxfordshire, in terms of identifying individuals that are wintering in the county - over 30 at the last count. I haven't found a new bird for several weeks (although doubtless there are probably some at Appleford), and combined with the lack of movement typical of January/February it's been a case of seeing most of the same birds again and again. Our regular 2w bird, which Adam has dubbed "Eric" has been roosting most evenings, together with a few other birds of various ages. 

"Eric" together with the pale 2w argentatus

Despite this, the roost has remained interesting with an almost continuous run of wacky hybrids, identification puzzles and straight-out "weirdos" that make gull watching such an entertaining form of birding. It is these birds that form the basis of today's blog post.

Starting on familiar ground, it was good to see XJNE back to terrorising the roost this week. It was also joined on one evening by two other Caspian x Herring gull hybrids, including remarkably a bird rung at the same Braunsdebra colony on the exact same day! A strange coincidence, although the birds came in separately and did not interact at all. Great to see the range of phenotypes that this mixed colony is throwing out - XJNE is a really nice bird, albeit with heavily marked greater coverts.

In stark contrast XJNJ is a real piece of muck, an obvious hybrid with clear cachinnans influence in many traits, with the scapulars, greater coverts, tail and underwing in particular being intermediate - it also has quite a thin bill and rangy structure - but other than that looking rather like a Herring gull. 

This third bird is very Caspian-like, and at distance in the Farmoor roost, for example, I think I would probably just have identified it as a dark Caspian gull. It has advanced moult with many replaced coverts and nice clean scapulars, coverts and tertials - all good features for Caspian gull. The underwing is quite streaky, as is the head, but these features can also be found on Caspian gull from the core range. Where it is less convincing is its structure and facial expression, although this was not immediately obvious at range as the bill is quite thin. I always find it confusing assessing facial expression on birds like this as any kind of head streaking changes all the contours of the face making it very difficult to tell if that classic cachinnans facial structure is still there. Having looked at photos of dark birds from Eastern Europe I struggle to reconcile the facial expressions of these birds with what I am used to from the birds that I have seen in Britain. In addition, the bird is quite small, and I do find that these (presumably female) birds can have very short bills and punched-in faces - see the last image in the set below for comparison and also this bird on Josh Jones' blog. However, I feel that this latest bird definitely has a very Herring-y look to the face, which is particularly evident in the third photo.

For comparison a presumed female bird with a tiny bill from February 2019

A new 2w bird has also arrived in the roost and for me is a good candidate for a Caspian x Herring gull hybrid, having a really Caspian-like facial expression combined with extensive head streaking, especially around the eyes. The tertials and greater covert patterns are an intermediate type, which could theoretically be found on either Caspian or Herring gull.

This dark, heavily streaked bird is probably a Herring x Lesser Black backed gull hybrid - with the added feature of a massively over-hooked bill! Its mantle shade was similar to Yellow-legged gull. It also has a yellow colour ring, although it's been too far away to read the code.

After the recent run of Easterlies I have been looking for yellow-legged Herring gulls in the roost, i.e. those "omissus"-types that originate from the Eastern Baltic. With several reported on the East Coast I thought it would only be a matter of time before one made it to the Meadow - small rewards, perhaps, but a goal like this keeps one going. I picked out this bird on 19th February and immediately thought it looked like a good candidate - the lack of any black on P5 would appear to rule out Yellow-legged gull, and the mantle also looked slightly too pale for that species. It had a strange, asymmetric primary pattern, with a complete P10 tip on the right wing (unusual in Yellow-legged gull) and a thin subterminal band on the left. However... this bird lacks the long P10 tongues typical of "omissus"-types and its head also just looks exactly like a Yellow-legged gull - really mean expression and heavy, blunt-tipped bill with massive red gonys. I guess this is also possible on large, male Herring gulls, but this feature, combined with the lack of primary tongues (although admittedly still perfectly fine for argentatus) and also the mantle shade (lacking the bluish hue that I am used to seeing with argentatus) make this a far from straightforward case in my opinion. I wonder if it is a hybrid, which would explain its mantle colour and Yellow-legged gull jizz. Or perhaps it is just a large, male yellow-legged Herring gull and I'm being far too fussy about something that isn't even a discrete population any more - "omissus" was invaded by pink-legged argentatus decades ago, thus the classic yellow-legged phenotype is heavily diluted. Either way a fascinating bird. According to Chris Gibbins, Eastern michahellis can also have much reduced, sometimes completely absent P5 marks, combined with much longer primary tongues...

As an aside, this bird, which I saw at Appleford in mid-December, seemed to me to be a far more straightforward yellow-legged Herring gull, despite the white head. Ticks all the boxes - argentatus jizz and primary pattern, combined with yellow legs. Unless of course it's another Caspian x Herring gull hybrid!

All the above birds make me think more and more about what really constitutes a "species" in large, white-headed gulls. As a biology student, I find it difficult to regard a group of populations that can freely interbreed to produce fertile, hybrid offspring as "species" in any proper sense of the word. However, I don't think they should all just be lumped into one! Genetic differentiation between these populations is complex, and often at odds with the phenotypes that we recognise in the field. For instance - Iceland, Slaty-backed, and Glaucous-winged gulls can in most cases be easily identified in the field using morphology, but mtDNA sequencing reveals that there is very little genetic differentiation between these species, possibly due to their recent evolutionary origin or periods of hybridisation. In other cases, evidence from mtDNA implicates whole or partial reproductive isolation despite overlapping ranges and some recorded cases of hybridisation.

There is also a very interesting discussion about whether the clinal variation in large gulls is due to local adaptation, differential trait expression caused by phenotypic plasticity, or hybrid swarms in the contact zones between different "species" populations. Put simply, are the large, frosty argentatus with pale brown primaries (from the "far North") like this because of a local, slightly genetically distinct population, a gene-environment interaction, or the introgression of Glaucous gull genes? The latter is actually what has been concluded by at least one research paper (Crochet et al., 2003), i.e. that most intraspecific variation in gulls can be accounted for by hybridisation.

A third complicating factor is the expanding range of species such as Caspian gull, which is moving rapidly westwards and hybridisation with argentatus Herring gulls is now frequent. Many of these hybrids cannot be sufficiently separated from individuals that fall within the variation exhibited by Caspian gull from the core range - data from ringed individuals of known parentage has shown that some second generation hybrids are virtually indistinguishable from pure birds. However, there is a particular "look" about many of these East German and Polish birds, and I would argue that in itself this is a distinguishable phenotype - very often combining a Caspian gull jizz with rather Herring-like scapulars and heavily marked greater coverts - XJNE being a good example of this form. These birds also tend to have a streaky plumage and a slightly Herring-y facial expression. Ronald Klein, who works on these hybrid colonies in Eastern Germany has thrown the name "Larus polonicus" around to describe this distinctive phenotype. Perhaps in Europe we will soon have names for these hybrid swarms, in the same way that birders in the USA refer to Cook Inlet and Olympic gulls - both identifiable hybrid populations.

What does any of this actually mean for the British birder wanting to identify a Caspian gull for their list? Potentially, a larger number of confusing looking birds in the next few years - instead of dividing these down the middle into "makes the grade for Caspian" and "non-Caspian" perhaps we should be recognising these as a distinctive and increasing hybrid gull and only safely identifying the most classic, obvious "Pontic" Caspian gulls. This of course means that we will be chucking many non-classic "pure" Caspian gulls into the unidentifiable bin - but for birders who value the integrity of their species list then this is probably the best option. For me, since I think the whole species thing is far more fluid anyway I prefer to simply pick out "Caspian-type gulls" and not care about the exact genetic provenance of any particular bird. After all, a bird that may look completely classic may contain Herring gull genes somewhere down the line, which would be completely impossible to detect in the field unless the bird was ringed and its parentage known. That isn't really what birding is about for me - it's about being able to pick out identifiable, unusual phenotypes in the field. I just like the fact that they look different to Herring gulls and come from the East!

For this reason I found the extensive debate surrounding the 2011 Rainham Slaty-backed gull rather strange - although it was right to correctly establish the identity of the bird, from my point of view, it wouldn't have actually mattered if the bird had turned out to have had some Vega gull influence (although the mantle colour has now been documented as easily within variation). It's a brilliant Siberian gull that's travelled thousands of miles to be here, and an absolutely amazing find regardless of its exact ancestry! I fear that if a Cook Inlet gull were to turn up in the UK it would generate little interest simply as there is no tick box on the British List for "Glaucous-winged x American Herring gull" (and no chance of it later being promoted to full species status!) - despite being a mega rare American vagrant. I find the devaluing of hybrid birds, especially gulls to be quite a shame and it seems purely linked to a listing mindset. It doesn't change anything about the the find (or indeed the twitch!) - it would still be a spectacular, visually distinctive vagrant - yet just because no chance of an extra tick I think most would pass it over. I hope to be proved wrong if one does turn up!