Two nice finds

Northern Cross Valley Way

As soon as I've abandoned the patch it seems that I've begun finding birds again. Northern Cross Valley Way (a new site near Witney) has been attracting a run of good waders recently, including two Avocets on 26th April. I dipped these birds the next day and was pretty gripped by the potential of the site. It looks a lot like Appleford in its heyday, before it became overgrown - muddy scrapes just looking absolutely perfect for waders, all easily viewable from a raised bund overlooking the pit. This complex of lakes is also the first big area of water going west-east after the Cotswold Water Park, which no doubt helps to pull birds down as they migrate overland from the Severn to the North Sea.

On 4th May I arrived early in the morning and could see some small waders through bins on the far side of the pit. Setting up the scope, I began panning through them - Ringed Plover, Ringed Plover, Ringed Plover... Temminck's Stint. Nice. I'll admit I let out a little cheer. A self-found tick for me and only the second I've seen in the county after a frustratingly distant bird on Otmoor in 2017. It's a rather scarce bird in the county (last records being short-stayers in 2023 and 2018) and it's been a quiet spring, so unsurprisingly it generated a bit of a twitch - I think at least 30 people saw it throughout the day.

Initial distant views of the Temminck's Stint with a Ringed Plover, digiscoped at 60x magnification and heavily cropped!

First responders - Adam digiscoping at the front, courtesy of Justin Taylor

The bird then flew to the central spit and gradually moved closer, offering pretty good scope views as it crept around feeding. Still too distant for the camera even with the 2x extender, although I eventually managed to get this digiscoped image which I was happy with. After I left it apparently disappeared for a bit and was much harder to see amongst vegetation at the eastern end of the pit in worsening light as the day progressed.

Having been slightly rejuvenated by this find after a terrible effort/reward ratio over the past few months, I've been enjoying local birding a little bit more. I found a Black Tern at Appleford whilst doing a casual county day list by bike (not really a serious big day attempt - managed 104 birds and cycled ~150km) and enjoyed a couple of female Blue-headed Wagtails - sadly no males, nor rarer subspecies for me this year so far. I've also been making an effort to take more photos of common birds on my walks around Otmoor and Farmoor.

Black Tern, Appleford New Workings - the first in the county this year

Female Blue-headed Wagtail at Noke Farm, Otmoor

Note clear demarcation between olive mantle and blue-grey head, as well as pure white supercilium and throat

With a female flavissima

Barn Owl, Otmoor

Bittern, Otmoor

Marsh Harrier, Otmoor

Amazing light that evening but had to use high ISO levels!

Swift, Farmoor

Yellow Wagtail, Stanton St John

Gulls have been rather irregular at Stanton St John this year but given that it's only five minutes from my house I've been making sure to drive past and check it quite regularly. Early morning on 8th May I was please to see a large group of gulls swimming in the water and loafing around the edge of Shepherd's Pits - clearly having roosted the night before. Immediately I saw a nice 2cy Caspian Gull which is clearly different to the lingering Farmoor individual - a good bird in Oxfordshire in May.

The pictures below are the Farmoor bird for comparison - it's been around for quite a while and has now started its primary moult. A similar-looking hybrid has also been present and is causing some confusion!

Having checked most of the gulls on the water, I began searching through the more distant flock loafing around the field margins. There was a partially obscured Lesser Black-backed Gull that looked interesting, being obviously darker and smaller than the surrounding birds. It was sitting pretty tight but I decided to take a few record shots, assuming that it was an intermedius.

Initial views of the bird obscured in the grass

After a few minutes it began walking around and I was struck by its blackish mantle and structure - remarkably long-winged and attenuated at the rear end. I also realised that it was a subadult, with black markings at the end of the bill and only tiny white tips to the primaries. I began wondering if this could be a 3cy Baltic Gull candidate, although knew that I had to get a shot of the open wing to assess the primary moult. The bird looked unusually advanced (more like a 4cy) and I was struggling to age it with any certainty.

The flock was soon spooked and the whole lot took flight - luckily, I was still looking through the viewfinder and had the camera in position to take some photos, although with rather suboptimal settings for a flying bird (shutter speed far too slow and only the central focus point active)! Whilst the photos are not the sharpest, I was pleased to document the spread wing from both the top and underside. Examining them on the back of my camera, I could see that there was no active moult and two generations of primaries present - a clear indicator of 3cy fuscus!

Blurred photo but really shows the difference in size with this 2cy Lesser Black-backed Gull

I think the photo below shows the moult limit most clearly, with the outer two feathers (P9/10) appearing more brown and translucent against the background, as well the tips being slightly less rounded and lacking the small white tips present on P1-8. All secondaries are third-generation feathers. The contrast between the mantle and outer primaries is limited (in graellsii and most intermedius there is typically an obvious difference in colour), and there is a slight brownish cast to the greater and primary coverts which is a good feature for aging immature birds compared with full adults. Other than the dark eye, black bill tip and lack of P10 mirror the bird appears remarkably advanced - it has a fully white adult-type tail (3cy birds across all three Lesser Black-backed Gull subspecies usually show a mixture of white and black-tipped feathers), an unmarked white underwing without any marks on the underwing coverts, and deeply saturated yellow bare parts. Based on the size, thin bill lacking gonydeal angle and rounded head, I think this is an obvious female.

3cy fuscus is currently not acceptable as a vagrant by most rarities committees in Western Europe, including the BBRC. Both graellsii/intermedius can also moult primaries in winter and thus have two generations of primaries as a 3cy in spring. With fuscus this moult strategy seems to be the default, whilst in graellsii/intermedius it is uncommon but tolerably regular. The reasoning, then, is that since this moult strategy is not unique to fuscus, it is theoretically possible for an intermedius to show features which overlap completely with fuscus, including blackish mantle and small, attenuated structure. Having said this, I've yet to find any photographs of ringed 3cy intermedius which look exactly like this bird... if anyone has any please send them! Indeed, the examples of advanced intermedius used to illustrate these arguments do just look like advanced intermedius rather than birds that are easily confused with fuscus. This Norwegian bird (link) is the closest I can find. Typical 3cy graellsii/intermedius tend to combine grey scapulars with a worn, brown, unmoulted wing (sometimes with a row or two of coverts replaced) - they are instantly recognisable in the same way that 3cy fuscus is a striking bird.

I'd also argue that some 3cy fuscus must be identifiable if they have retained second-generation primaries with small P10 mirrors. Whilst this relates to only a small proportion of birds, as long as a candidate can be confidently aged as a 3cy this surely rules out graellsii/intermedius, which never (or almost never?) show mirrors on their second generation primaries. As far as I can tell, this hasn't been stated explicitly in any papers relating to fuscus identification, so the BBRC probably hasn't recognised it as a potential criterion for identifying 3cy fuscus.

Whether this latest Stanton St John individual is a textbook 3cy fuscus or an unusually advanced 3cy (or indeed retarded 4cy) intermedius - which also happens to share the full range of typically fuscus features - should be weighed up against the fact that confirmed 2cy fuscus are now known to be scarce but regular in Britain. I do think that the BBRC already applies a probabilistic approach to the assessment of certain rarities, where a combination of many "soft" features is sufficient to separate e.g. Zino's/Feas/Desertas Petrel, so long as these features are sufficiently documented through good photographs. I hope that this will soon be the case with Baltic Gull and I do plan on submitting this bird. I also think some clarity in the literature would help the BBRC with these complex assessments.

A bit of a deep dive now into Lesser Black-backed Gull moult, so an early point to bail from this blog post if one is so inclined!

One thing I found baffling after looking through lots of photos of 3cy fuscus was how many of them show large mirrors on P10 as well has having a moult limit in the outer primaries. Initially I thought that this was a sample of birds that happen to show mirrors in their second-generation primaries, now retained as worn outers. However, having a P10 mirror on second-generation primaries is a relatively uncommon occurrence - and some of these photos showed large mirrors (crossing over both feather webs) rather than the small and indistinct mirrors found on second-generation primaries.

The literature seems rather confused on this topic and I spent quite a while trudging through various papers attempting to explain why this pattern is observed. One school of thought seems to suggest that some birds which acquire P10 later on will show larger mirrors as the bird will be older and therefore produce a more adult-like feather. These may be individuals that arrest their first moult cycle in the spring of their 2cy and then replace the worn, juvenile P9/10 with second-generation feathers a few months after their peers. However, I'm struggling to find evidence that later moult leads to accelerated progression of feather pattern towards an adult-like pattern - indeed, I would have thought this would be correlated with earlier rather than later moult.

I found a more complete explanation from Chris Gibbins, who had obviously gone through the same thought process. His paper here (link) speculates that some of these birds have already moulted into third-generation primaries in their second winter, and those that show a moult limit have therefore replaced the inners with fourth-generation primaries. The most advanced fuscus may have already replaced half their primaries with third-generation feathers in the summer of their 2cy - so it is reasonable to imagine that the rest of these are replaced on arrival in their wintering grounds. Some Baltic Gulls would therefore remain a year ahead of other Lesser Black-backed Gulls in terms of their progression.

This makes some sense - it also seems possible that third-generation primaries are variable enough to show the full range from completely lacking a mirror to having a relatively large mirror. Indeed, the moult limit on the Stanton St John bird is very indistinct, with the retained P9/10 being barely browner than the inner primaries and not particularly worn/tatty. I find it hard to believe that these feathers are more than a year old, and have been used for three long migrations. Compare with the 3cy Lesser Black-backed Gull behind it - the second-generation feathers on that bird seem more worn than on the Baltic Gull, despite being grown a few months later. If you assume that the moult limit in the outer primaries in 3cy fuscus is between second- and third-generation primaries, the criteria for identifying 3cy fuscus suggested by Winters (2006) in his Dutch Birding paper seems to make more sense for intermedius! Perhaps this could be explained by the bird having replaced its P9/10 in the late summer of its 2cy (i.e. a slightly slower-moulting fuscus).

It is unfortunate that the third and fourth generation primaries (especially the inner ones) look very similar on Lesser Black-backed Gulls, which means that it is hard to reconstruct the moult progression based on feather pattern alone. I would imagine that 3cy fuscus shows a lot of variation in moult, with some birds having a mix of second- and third-generation primaries and particularly advanced birds having fourth-generation inner primaries. Indeed, this is the conclusion of this article on gull-research.org (link - a fantastic resource) - scroll down to see a list of observed moult strategies in 3cy fuscus returning to Finland. The moult pattern shown by my candidate bird is described as the "common strategy". I guess then, the problem is how to age an unringed bird accurately in the first place, given that bare-part colouration is more variable - and then how these differ from 3/4cy intermedius.

It's possible that I have misunderstood all this completely so if anyone has any comments that would be greatly appreciated. Clearly I need to visit some fuscus colonies this year and get some experience looking at birds of various ages within their core range - again, looking for any recommendations for good sites!

Good day at Farmoor

Arctic Tern, Farmoor reservoir

Wednesday 23rd April looked like a promising day for birding. The brisk easterly wind and rain predicted throughout the morning are classic conditions for overland passage, particularly of waders and terns. I had been gripped by the quality of birding at the big Midlands reservoirs (e.g. Belvide and Draycote) over the past few days, with Oxfordshire having seemingly missed out on all of the action. With early reports of Arctic Tern movement from Staines I was hopeful that this weather front would finally produce some good inland birding in the county.

I arrived at Farmoor in the drizzle just after 8am, and as I walked past the cafe I saw a few hirundines and two Common Terns flying over the causway. Peering through the murk across F1 I could just about make out a large flock of terns flying around in the northwest corner of the reservoir. At least some of these were Arctics from the flight style, although I was struggling to see any details - I had to get closer.

Initial views of the first Arctic Tern flock

A very wet Rock Pipit briefly materialised as I hurried along the causeway - a year tick for me.

Upon reaching the hide I briefly took shelter from the rain and started looking through the distant flock - most of the birds appeared to be Arctic Terns (at least 30) but there were also three or four Common Terns, although these seemed not to be associating too closely with the Arctics. Counting the terns from next to the hide, I saw a second large flock drop out of the sky and fly right past me onto F2 - another 45 Arctic Terns! Absolutely fantastic to see inland, and really exceptional numbers for Oxfordshire. I was glad that my hunch with the weather had finally been correct - I guess a stopped clock is right twice a day.

At this point I had to go to an opticians appointment - luckily it was in Botley, only ten minutes away, so I just about managed to tear myself away from the action and headed back to the car park, meeting Dave Lowe and Conor MacKenzie as they arrived. The rain was forecast to continue for much of the morning so I hoped that everything would stay put and indeed that more birds would be brought down. I anxiously waited out the next hour getting trialling some contact lenses before returning to the reservoir at around 10:30. Luckily, it seemed that I hadn't missed anything although the number of birders standing on the causeway had grown considerably!

My first Swifts of the year screeched overhead as I arrived back at the hide. The light had improved considerably and the terns were much closer than before, so I began taking some photos. Most of the shots had white and grey birds against an equally dull grey background but I managed to get a couple where the subject was isolated against the green trees bordering the reservoir. A bit tricky with the changing backgrounds and gloomy lighting, but I can't complain - easily the best photos of Arctic Terns I have ever taken inland.

As I was watching the Arctic Terns another 20 or so flew straight over F2 very high up, powering into the headwind. With around 70 already on F2 and another 30 on F1, this brought the day's total to at least 120 individuals. Who knows how much turnover there actually was throughout the morning!

High-flying flock of Arctic Terns

A surge of activity amongst the assembled birders indicated that something else had just been found - walking along the causeway to join them, I saw that a Grey Plover had pitched down and was feeding along the debris at the reservoir edge. It was flushed several times throughout the morning, each time flying high northeast, but always ended up circling round and coming back to the causeway.

I was vaguely aware of someone shouting whilst I was photographing the Grey Plover although it was only when I looked up that I realised that Dave was waving frantically at me - panic! I glanced over the reservoir and couldn't see what they were looking at. Running back down the causeway, the bird of the day was revealed - a fantastic Little Tern which Dave had picked out as it flew in to join the Arctics.

Dunlin and Ringed Plover were arriving throughout the morning, including a fantastic flock of 15 Dunlin that made several close passes. A few Yellow Wagtails and a White Wagtail also dropped in.

Whimbrels occasionally flew over (I think I counted four throughout the morning, although others had more). Again a classic time of year for this species.

Whimbrel low over the water, mobbed by a Herring Gull

These two Bar-tailed Godwits were perhaps my favourite wader sighting of the morning. They went straight through northeast - two were also seen at Otmoor later on, which I suspect were the same birds.

As the rain stopped and the weather improved, the flocks of Arctic Terns began to rise higher and depart. By lunchtime, only around 20 were left, along with the Little Tern. At this point, I wasn't seeing much more in terms of newly arrived birds so decided to go and do some work.

Having missed the "Arctic Skua day" at Farmoor last September (due to being on Portland), it was good to finally get in on one of those rare occasions where birds were just piling through and dropping out of the sky. Certainly one of the best days of county birding that I've ever had.

Arctic Terns departing Farmoor and continuing on with their migration