​Lee Fuller and I had been talking daily  about a potential twitch ever since the Tengmalm's Owl first turned up on Shetland. With the bird having relocated to Tresta and now showing daily, we headed up with Lee's dad Mick and Ian Wells on Thursday, taking the evening ferry sailing from Aberdeen.

​It didn't take too long for Lee and myself to relocate the owl roosting in an evergreen bush on the opposite side of the garden to where it had been favouring in the previous days, largely thanks to the panicked calls of the Blackbirds and Chaffinches that had discovered it, which allowed us to identify the right bush. Initial views were rather obscured, but we were just pleased to have seen it and could now take solace in the fact that we knew where it was for the hoped-for evening showing.

​We took a trip around various sites in South Mainland, seeing various bits and pieces including the resident Pied-billed Grebe at Loch of Spiggie, several Long-tailed Ducks at Grutness and two Tundra Bean Geese at Leebitten. This was my first Shetland visit outside of the migration season and it felt a bit strange to be driving past garden after garden with complete disregard! However, the weather was just about as nice as I ever remember enjoying on the isles – conditions were bright and still, complete with the occasional burst of early-spring sunshine, making it a very pleasant day indeed.

​With reports of multiple white-winged gulls at the Shetland Catch fish factory in Gremista over the previous weekend, we were keen to give the site a go. Unfortunately, despite offloading six loaves into the waters at the pier there, just two juvenile Iceland Gulls made appearances and neither seemed particularly interested in the wholemeal offerings. It was in fact the Kittiwakes that provided the best larid entertainment. At least the local Black Guillemots were very confiding, albeit not particularly great for photos in the flat light.

Back at the owl and, completely unsurprisingly, we found it sat in the exact same spot. It wasn't until around 17:30 that it became more active, looking around in alert fashion, preening, stretching, and eventually hopping out onto a more exposed perch, where it sat for a few minutes before flying off past us and into the main garden. Cue much celebration among the 30 or so gathered birders, before the crowd began to thin out. As it turned out, most birders walked right past it sat just above head height in a sycamore, right by the access track. I took a couple of record shots, but it just would not look our way, meaning my best Tengmalm's photos showed the back of the bird's head. Excellent.

After a celebratory curry in Lerwick and a welcome night in a bed, we returned to Lea Gardens on the Sunday morning, where a dozen birders were already looking for the owl. It quickly became apparent that a mass frog orgy had broken out across the gardens, with hundreds of vocal adults all over the lawns and flowerbeds, and huge piles of spawn being laid in the pond. Unfortunately there were a few under-foot casualties and the owners' dogs also took to mercilessly removing the heads of some of the hapless amphibians ...

Some time passed before the sound of an alarm-calling Blackbird saw us hone in on a bush, where we figured the owl must be. A few minutes of careful scanning and Lee once again picked it up, extremely obscured, in the middle of the bush. Cue the typical rush from birders yet to connect, excitedly lapping up just about anything they could decipher from amid the foliage. With the bird seemingly very much buried in the bush and fast asleep, we decided to leave the newly arrived birders to it and head off for pastures new.

We headed up to South Nesting Bay, where the White-billed Diver was eventually picked up at extreme distance (its bill was barely visible on a grey day). With the views leaving much to be desired, we headed back south to Lerwick harbour, although the primary excitement there proved to be a very confident Grey Seal that spent time playing 'fetch' with the slices of bread I was throwing out for the gulls. A third-winter Iceland Gull did make a brief appearance.

We made another trip south for the wintering Common Rosefinch at Cunningsburgh, but managed to dip it again (though I can't say I was particularly gutted about this), before returning to Lea Gardens for the fourth and final time. It was a pleasure to talk with owners Rosa and James, who clearly take great pride in their work here and have done a wonderful job in developing the gardens into what they are today. You have to wonder how many rares have been missed in here over the years – the habitat is fantastic. Anyway, back to the owl, and it was a pleasant surprise to find that it had shuffled out into the open and was showing very well indeed, as close as four metres away (if you were in the right position along the path).

By chance, one of the owners' dogs came over to the owl's favoured spot and started rooting around in the flowerbeds below it, generating quite a reaction from the bird. Although completely unphased by humans, this bird takes dogs to be a more serious threat ...

Although the bird wasn't particularly happy about the dog's presence, it's fair to say that the crowd was. The pooch's comings and goings ensured that the owl would regularly stir from its slumber and look down towards the crowd peering back up at it, its look varying from mildly irritated to completely and utterly seething.

The above photo is barely cropped and was taken at 135 mm focal length, just to illustrate how close this bird was to the assembled 15 or so birders. An amazingly intimate experience and one that will live long in the memory – and yes, I have to admit that, in contrast to a recent tweet of mine, the owl is most definitely as good as a Caspian Gull. Needless to say it was a high-spirited ferry journey back to Aberdeen that night.

Many thanks to Lee for driving to and from Aberdeen and Ian for organising the logistics on Shetland. Everything went very smoothly and was in good company.

I had the day off today, but ignored the temptation to travel north to Shetland for the Tengmalm's and instead did the usual tour of the regular East London sites.

It was pretty foggy early on and I couldn't even see the river when I arrived at the stone barges car park at Rainham, so headed back west to Creekmouth. Although the fog lingered for much of my visit here, it was fairly mild and around 10 Chiffchaffs were seen, though there wasn't anything among the gulls at the sewage works and outfall.

With the fog starting to clear by mid-morning I headed back to Rainham, where I soon picked up the juvenile Glaucous Gull floating around over the tip. Most of the gulls were going down to bathe on the river by the barges, so I headed up there. Viewing conditions were actually pretty good and it didn't take long for the 3cy Glaucous to put in an appearance, as well as a 2cy Yellow-legged Gull.

By late morning the haze was worsening and the temperature rocketing, so I headed back to the car and round to Erith Pier. Erith seems to be more of a weekend site, although gull numbers do vary with each visit. It was pretty quiet again today on the rising tide, with the six loaves dispatched into the Thames luring in very little. I'm still living in hope that the Bonaparte's might reappear, but it seems pretty unlikely now ... anyway, with nothing on show, it seemed churlish to ignore the fantastic light opportunities and I rattled off a few images of the Herring and Common Gulls drawn to the melee. I also saw the leucistic Herring Gull very distantly on the river towards the stone barges.

After establishing that Erith wasn't on song I made the short journey east to Crossness. A Eurasian Penduline Tit has been seen regularly this week, after initially being found back in November. It was showing on arrival, but was a bit distant for images ... although did eventually come closer, if only for a minute or so, before flying off out of view. It was nice to hear it both calling and (briefly) singing!

With the sun beginning to dip, I turned my attention back to gulls and headed for Princess Alice Way, Thamesmead. The tide was right up, and the customary wholemeal offerings soon brought in the usual handful of gulls. Given how few Herring and Lesser Black-backs are usually here, this spot has had a fantastic strike-rate with Yellow-legged Gull this winter and today's visit produced a new first-winter that I hadn't seen previously.

Where do you draw the line between one species and another? That is so often the burning question when it comes to large white-headed gulls.

Not only does each species tend to show great intraspecific variation in its physical characteristics, but species also routinely hybridise, further clouding the issue and eroding the recognised parameters between defined taxa. Perhaps the most recognisable example of a so-called 'hybrid swarm' among Palearctic gull-watchers is the Iceland-Kumlien's-Thayer's 'spectrum' breeding across Greenland and Arctic Canada. Of course, these are now apparently to be considered as one and the same species, but it was nonetheless a fine example of how a man-made scale, assessed on the basis of quantifying several physical features, was – rightly or wrongly – used to attach a label to any gull in question within the complex.

To some degree, the same 'problem' is rife in Central Europe. In parts of eastern Germany and Poland, and indeed to a lesser extent elsewhere, Caspian Gull has, on its westward march, come into routine contact with European Herring Gull, which itself has spread inland in recent decades. These two species seem to have no qualms about breeding with each other, as the numerous mixed colonies testify, and hybridisation between the two is very well documented through intensive studies by dedicated researchers, including long-standing ringing schemes.

Whether it's a case of increasing observer awareness or a genuine upturn in the frequency and number of hybrid pairs – or, more likely, a mixture of the two – birds exhibiting mixed characteristics seem to be on the increase in Britain. These birds show any number or combination of features of both parents, both in terms of plumage and structure. Perhaps I'm too critical with my Casps these days, but it seems to me that I'm seeing more and more of these 'unimpressive' birds – albeit a longer-term assessment of well-photographed individuals is probably sensible before leaping to surefire conclusions.

This was illustrated pretty well by the weekend's Thames gulling. On Saturday, I had a rather nice-looking first-winter Casp at Princess Alice Way, Thamesmead. It transpired that this bird, with its particularly distinctive look to the face, had been seen by Jamie and Dante at Crayford back on 31 December, but not since (Dante recognised it immediately). OK, no moulted coverts and perhaps a slightly mucky underwing, but plumage otherwise absolutely spot on for a textbook first-winter Casp.

Sunday, however, was enlivened by a couple of troublesome birds, one of which I have subsequently described more than once as a "horror show". It is perhaps no surprise that both bear German rings, to the point that Rich actually said of the first (X21A), "I bet that has a yellow ring on it" before it revealed its legs. Sure enough, it did. In terms of plumage, though, X21A – seen at Erith Pier – was the better of the two. Apart from its well-marked greater coverts, it otherwise looked fine, but structurally lacked Casp qualities in many poses. It was quite vocal, and sounded like a typical Caspian Gull.

However, if you thought X21A was bad ... then allow me introduce you to X90A. From the same colony as the above, this confusing mess of a Larus again sounds like a classic Caspian Gull, with the hoarse call often announced as this bird throws its weight arounds. And, at many angles, looks good for a cachinnans, with its rangy structure and various plumage features looking fine. But just take a look at those greater coverts, the extensive inner primary window, the well-marked tertials, the Herring-like scapulars, the comparatively uniform (and extensively pale-fringed) upperwing, the streaked head, the tail pattern ... suddenly you realise things aren't exactly looking great for a pure Casp. And, when you see it on the deck, it's enough to make you have nightmares.

I had to pop up to Lincolnshire at the back end of the week, which gave me a small window to get some local birding in. Although I'm sure the cold snap probably helped a bit, I came back to London really enthused by winter birding in the area – it's only when you get back to the murky grey streets of the capital, where gulls are the primary order of the day, that you truly appreciate the wider avian opportunities of the Cambridgeshire and Lincolnshire fenlands.

That said, I nonetheless tried the gulls at Tanholt first thing on Friday. Actually it was overwhelmingly disappointing for larids, with large gull numbers low at the landfill site, where there wasn't much tipping activity going on. However the Hooded Crow was showing on arrival, although it dropped out of view before I could get my 'scope on it and it was only about 45 minutes later that it popped back up. Quite a smart bird really.

A quick zip around the Deepings area brought good birds at all sites visited. Deeping Lakes must now be the 'best' (at least most reliable) Long-eared Owl roost in the country, with five birds showing on the island on the main lake during my visit. Subsequently, six have been counted. It's come a long way since these were intermittent visitors to the nearby 'Gulley' area, although we have found nests of the species here in the past.

Deeping High Bank was also alive with birds, with upwards of 30 Goosanders along the Welland itself and a couple of separate flocks of Whooper Swans amassing to around 25 birds. I was happy to find a family of five Bewick's (the three juveniles were particularly pleasing) towards Deeping St Nicholas, while another 'family' (an adult and a juvenile) had arrived by Saturday. Marsh Harriers are also an ever-present fixture over the fields here these days and a Pink-footed Goose was with Mute Swans on the Friday.

My old patch at Baston & Langtoft Pits always has a few birds of interest, and a couple of visits on Friday and SAturday produced an adult Bewick's and up to a dozen Whooper Swans – the former being the first I've seen here since the big freeze in 2009/10. Even better still was a flock of 15 Eurasian White-fronted Geese, which looked pretty epic feeding among the wild swans on the frosted fen.

The new year commenced in very much the same manner in which the previous finished, with gulls providing the staple for winter birding along the Thames in East London. The 3rd produced a couple of Caspian Gulls at Crayford, followed by a couple of first-winter Yellow-legged Gulls off Erith Pier. The obligatory brief stop at Princess Alice Wharf, Thamesmead, produced a further first-winter Yellow-legged, this bird being quite distinctive for the amount of covert moult it had already completed.

Dante S scored hefty totals of 12 and 17 Caspian Gulls at Rainham Tip on Tuesday and Wednesday respectively – both exceptionally high counts for this site, with the latter in fact comfortably beating the previous record single-site count (of 13) from the London area.

With Thursday morning free, I met up with Dante and we headed over to Rainham for 8 am. It was a grey morning with bouts of drizzle and a strong southerly wind that was gusting up to gale force at times for a couple of hours around lunchtime – not exactly optimal for viewing along the exposed riverside footpath towards Coldharbour Point. Nonetheless the Casps soon started flowing with a flyover first-winter quickly followed by three more on the river itself, including a particularly fine bird with moulted coverts and tertials on the rocks by Coldharbour Point lighthouse, pictured below.

The setup is currently pretty good at Rainham. Veolia are currently actively tipping on the river side of the landfill site, which means the birds are more inclined to wash and loaf on the river. While the tip face is a tad distant to view from the riverside footpath, the views are generally pretty good when they come down to the river itself, usually just to the east of Coldharbour Point. At low tide, there are usually a few birds along the foreshore as far as Aveley Bay, with a roost of up to a few hundred birds just west of the lighthouse. Add the fact that there is plenty of turnover as birds commute between the tip and river and it's reasonable to assume that you'll be getting views of a good proportion of the birds present if you're willing to give it long enough. It's worth pointing out that numbers are by far best on weekdays, with Saturday mornings much quieter and Sunday more or a less a write off.

Our Caspian Gull total steadily rose throughout the morning and we were in double figures by lunchtime, despite the odd half-hour lull where few birds were using the river. In the afternoon were joined by Rainham stalwart Andy Tweed, with the three of us ensuring good coverage of the large numbers of birds commuting between the river and tip face as the afternoon wore on. I don't think we were missing any birds that came down on the river itself, but of course it's always difficult to gauge just how what percentage of the birds you've looked through – bear in mind that we were still seeing new Herring and Lesser Black-backed Gulls by the time we left (recognised by distinctive stains, plumage etc).

When Dante picked out a smart second-winter on the river a little after 3 pm, it signalled the end of a memorable session. Our final total of 19 individuals was very much conservative. We managed some sort of record shot of at least 12 of these birds, with detailed plumage and structural notes taken for the remainder. Some individuals were definitely seen more than one, while we also had sightings of birds which may (or may not) have been different that ultimately did not count towards the final total. The breakdown was as follows: 13+ first-winters, two second-winters, two third-winters (both birds seen on our trip the previous week), a fourth-winter and an adult. A few terrible phonescoped shots of some of the birds follow in a slideshow ...

I'm not sure what the British record single-site count is for Caspian Gull (there was a report of 23 from Rufforth, North Yorkshire, a few winters ago, but I'm not sure if this was ever verified), but surely 19 must one of the highest ever made? Answers on a postcard please.

As for Rainham ... well, this is something of a renaissance for a site that Dominic Mitchell has occasionally mourned as nigh on finished while talking gulls in the Birdwatch-BirdGuides office in recent months. In the three chastening days Dante spent on the riverside footpath, he notched up a minimum of 28 different Casps (again conservative, with the figure more likely at 30+), which suggests that a day count exceeding 20 is comfortably within the realms of possibility if the stars align. Perhaps there's life in the old dog yet – as long as birders keep chucking their food waste in the general household bins, there'll be at least some food going into landfill for the gulls!

I didn't see the well-watched Caspian Gull at Eagle Pond, Snaresbrook, last winter, so was pleased to hear that Jamie Partridge had picked it up again on Sunday. I went over to see it at lunchtime today, meeting Jamie there, who'd helpfully brought a couple of loaves with him.

The structure and demeanour of this bird looks identical to a first-winter that spent winter 15/16 at nearby Wanstead Flats (photos here) and is thus presumed to be the same individual which has subsequently taken up winter residence at Eagle Pond. It looked really good in its first winter and looks fantastic now; it would have been interesting to see this bird in second-winter plumage, which is sometimes the most distinctive of all, as I suspect it must have been a real beauty.

Curiously, the Casp is often the only large gull on the pond. There are occasional visits from the odd Herring or Lesser Black-backed but the Casp was the only to hang around while I was there, bullying the Black-headed and Common Gulls over the bread offerings. It was always on show during the hour I spent on site, although it does apparently wander to nearby Hollow Pond on occasion.

I've uploaded a gallery of images from my May 2015 trip to Azerbaijan to my photo pages. You can navigate there yourself via the Photography section in the header of this page, or alternatively click here or on the Blue-cheeked Bee-eater below.

I had a quick look at the Red-necked Grebe at Ferry Meadows this lunchtime, but it was festering away in the middle of Overton Lake and I didn't really have the time to hang around and wait for it to show any better. With a message from local birder Andy Frost suggesting that one of the two Rough-legged Buzzards was showing well at Holme Fen, on the south side of Peterborough, I headed straight down there.

Rough-legs are traditionally that sort of bird that you can spend hours looking for and never actually see particularly well, so it was nice to see both juveniles in the Holme Fen area with minimal effort, one of which showed delightfully over rough ground by the B660 – in fact, the views were probably the best I've ever had of the species in Britain. They're absolutely brilliant birds and it was really nice to enjoy prolonged views at close range and appreciate just how different the species is to Common Buzzard – not only in terms of plumage and structure, but also flight action, which is almost more akin to Red Kite, with flat wings, a 'bent' forewing at the carpal joint and the distinct tail twisting as it floated on the breeze, not to mention the characteristic hovering when hunting.

The past week has produced a big push of Caspian Gulls into eastern areas, with fantastic counts including 14 at Dungeness, Kent, thus it was no great surprise to see find one at Cromer this morning. The bird appeared pretty much instantaneously as soon as I started throwing bread, initially located by its distinctive, harsh call.