Common Gulls Larus canus start arriving in Osaka in mid-October, with some present till mid-April. I might see 30-40 birds per visit to the Yamatogawa (Yamato River) in Osaka during winter but daily counts number in the hundreds during March. Many over-wintering gulls associate with Black-headed Gulls Chroicocephalus ridibundus on the river but most, especially during migration, only spend a short time on the river each day, dropping in to bathe, preen and loaf before heading back out into Osaka Bay. Many easily recognisable individuals are rarely seen more than once indicating a high turn over during passage and suggesting a huge number of gulls could be passing through the region during March. Some also winter on Lake Biwa making it the only taxon other than the widespread Black-headed Gull that can be reliably found away from the coast.
Many people comment on the bewildering difference in size and structure between the largest, most robust gulls and the very small delicate birds. The largest approach small Vega and the smallest aren't so much bigger than Black-headed. The big questions are if, or more likely how many heinei occur, how much of a potential problem kamtschatschensis x heieni intergrades might be given their apparently extensive overlap in breeding range according to GULLS (Olsen et al) and whether this potential presence blurs any morphological boundary that might otherwise be more apparent between the two taxa? Additionally, there are many claims of Mew Gull L. brachyrhynchus in Japan and while they might be expected to occur would any other than the most extreme individuals even attract attention? I have plenty of questions and no answers, the images below show some of the variation I see in Osaka.
The obvious starting position that the largest, heavily built birds are kamtschatschensis is hard to argue with but any expectation that the smallest, lightly-built are birds good contenders for heinei doesn't follow. There's no shortage of mini-kamtschatschensis around as the presence of a small number of apparent 1CY heinei compared to the large number of equally small very dark 1CY/2CY kamtschatschensis attests. But when it comes to adults it doesn't seem possible to differentiate between them. The additional problem of the potential intergrades, which isn't possible to prove one way or the other, means an unknown number of birds below might not be kamtschatschensis. Starting with saddle shade, there's quite a bit of difference between the lightest and darkest though the very darkest aren't common.
An obvious kamtschatschensis with three Taimyr Gulls (part of a flock of about 30), the large size and very dark saddle of this individual leave no doubt about this gull.
Quite a range of saddle tint here, the gull on the left is another example of the uncommon very dark individuals.
Less contrast between these two respective pairs, it's no more than coincidence that six days apart the upper duo have stronger bill markings and more complete non-breeding heads.
Moving on to the non-breeding foreparts, obviously earlier in the winter head markings are more prominent, they are variable in extent and intensity but are reasonably consistent in basic pattern. Usually the feathering along the bill is unmarked, the crown finely streaked often with denser markings around the eye and/or ear coverts, the hind neck becomes more spotted compared to the crown and this frequently coalesces into a smudgy base. Below is a range of head, neck and breast patterns, some less common than others but none can be considered unexpected if spending any time watching these birds.
Two fairly heavily marked with typical patterning, one with a narrow bill band and the other unmarked.
A typical bird in the background and a heavily marked gull in front. The feathering along the top of the bill is often combined with darker, denser than usual markings elsewhere on the head and neck. Taimyr Gull on the right.
Another dark gull on the left, such extensive breast markings are less common.
The neck and breast markings are more extreme on this one.
Dark heads render eye crescents more prominent where they exist but, unlike this gull, many don't show this feature.
So far the gulls shown have heavily peppered dark-looking eyes but many are more lightly marked.
The gull on the right is also probably a third winter and again has a complete bill band and also very small A-spots and green legs.
This gull is genuinely unusual, the combination of large head and short, stout bill along with short primary projection and exceptionally large p10 mirror give it an almost mini-Vega feel.
A lot is made of wing-tip pattern as a means of identifying the different forms of the group but birds here show such a degree of variation it seems unwise to put much faith in it. If there's such a thing as a typical kamtschatschensis the outer primaries should look something like this; p10 has a short grey wedge on the inner web angling from the base to the edge on the underside but this isn't visible on the upperside. The upperside of p9 often does show a small area of grey in the base though at a glance it seems all black, however the underside can have a grey tongue up to a third of the feather length. P8 is more variable, typically the upperside tongue is slightly longer and forms an neat continuation from p9 across the outer wing, on the underside it is frequently two thirds the feather length. P7 has a long tongue above and below with a prominent white tip. P6 has a broad black sub-terminal band extending a short way up the leading edge. Whereas the band on P5 is narrower and of even breadth. However there are plenty of exceptions.
The two flight shots above illustrate the typical kamtschatschensis wing tips with consistent p9-10 mirrors. The bird on the right in the lower shot has a very small p8 mirror, they are often much larger.
There's probably no such thing as a perfect example of a standard Common Gull but it might be difficult to find one closer than this, even though the p5 band is broken.
At a glance this is another typical gull but there are subtle differences from the previous gull. The broken band on p4, very narrow white tips to the inner primaries and bleeding rather than sharp divisions between the grey and black of the outer primaries are all atypical features to a certain degree but each crops up regularly.
March 20. Another with a narrow trailing edge to the inner primaries, minimal white separators and relatively short grey tongues in the outer primaries, basically a typical "dark" bird.
An incidental video grab that appears to show a Common Gull with a white-tipped tongue on p9. Unfortunately I didn't see this bird in the field but the stretching wing really stood out watching the tape.
A long tongue in p10; this is beyond unusual!
The bird on the left has a long p8 tongue with a long white tip terminating close to the proximal edge of the p9 mirror. The bird on the right has a shorter white-tipped tongue but it also has a prominent third mirror combining to produce a very white wing tip. Despite getting many shots of the bird in the middle, its identity remains a total mystery but getting two whiter wing-tipped gulls in the same shot by chance shows they aren't so unusual.
Most birds have the proximal edge of the p10 mirror angled towards the edge.
Some gulls have the p10 mirror proximal edge squared across the feather.
An example of a very small bird and in direct comparison the bill was no thicker but obviously shorter than those of Black-headed Gull. In flight it (below) it was very white-winged but this was mainly due to a prominent p8 mirror.
A closer example of a bird showing a bold string of pearls. Though this feature is associated with Mew Gull, it isn't rare in the Osaka population.
Comparison of wing tips, a brachyrhynchus pattern next to a typical bird.
If the above outer primaries are more associated with brachyrhynchus than kamtschatschensis, the following bird, one of two others in the same flock as the above bird, is altogether more Mew-like. It (they?) could even be Mew, if not they certainly complicate matters.
The bird in question (top right) shows much white in the primary projection because of the extremely long white-tipped tongue leaving only a narrow black sub-terminal band.
The head differs from the norm in have such indistinct markings to be largely grey-washed
While some kamtschatschensis are small-billed this bird is particularly so.
P7-8 look completely grey for most of their length with extensive white longer than the sub-terminal black which bleeds only a short distance up the edge of the outer web. The apical spots on p6-8 are noticeably small than I'm used to. The final very heavily cropped shot also shows the soft, indistinct head markings and a more pronounced choker than shown by kamtschatschensis.