Wednesday, August 18, 2021

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...

4 comments:

  1. Nice post Thomas, enjoyed reading. Wondered what you thought of the bird linked below I had in 2019, which after much conferring online was generally regarded as a 3cy Intermedius?

    https://www.facebook.com/groups/europeangulls/permalink/1678393768959879/

    or

    https://twitter.com/DLangstonBirder/status/1160479506399997953?s=19

    Thanks for any thoughts.

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  2. This is high quality Tom. Probably too high quality for me to fully appreciate but I actually feel is if I need to see this Gull now!

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  3. Superb. Thanks for re igniting my, dangerously, dormant interest in gulls Thomas

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  4. Excellent stuff Thomas, enjoyable reading and very informative.

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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 ...