Kamtschatschensis is well known as the largest taxon within the Common Gull group, many are almost as large as Taimyr Gull or a small Vega. However the size range is enormous and in fact many are little bigger than Black-headed Gull.
The following images gives no more than a rough idea of some of the variation I see here.
January 3. Full juvenile, unsurprising in mid-winter.
March 8. Still in juvenile plumage but looking very faded as might be expected at this date.
March 15. A barely faded full juvenile in late winter.
April 1. A slim, long-winged juvenile, still quite fresh looking for April!
March 22. A heavily-built very dark individual.
The above, unfortunately rather distant images, are a selection of clearly kamtschatschensis juveniles from early January to April. Of course an increasing number of birds develop a grey saddle and white head during winter but full, or near full juveniles are frequent. In contrast the images of the following bird, present on much earlier dates November 12-21, when kamtschatschensis are invariably in full juvenile plumage, is a good example of (presumed) heinei. It's a crisply marked bird which has already replaced mantle and upper scapulars with adult-grey. Plumage differences include the largely white underwing, particularly wing lining and axilliaries, clean white rump and tail base with an isolated, narrow black tail band. Correspondingly the vent and undertail coverts are only lightly marked, again looking very white in the field. I believe this probably is heinei and it's interesting that it's the only such bird I've noticed implying this isn't a common taxon in Osaka.
November 21. Strikingly white underwing; the lining, median and greater coverts have light brown restricted to tips, the inner axilliaries have the same light brown broken bars on the innermost feathers reducing to two pairs of subterminal spots by the central feathers while the inner half has a single pair of subterminal spots. The greater coverts looks very clean and the window contrastingly open.
November 21. The still fresh longer lesser coverts have triangular dark centres and white fringes, the greater coverts are basically pale grey, early kamtschatschensis have much browner greater coverts which gradually fade greyer. Compared to many kamtschatschensis the bill is shorter, spikier and less deep measured against the eye but there is enormous variation within apparent kamtschatschensis.
November 19. The same bird in bright sunlight giving it a warmer look. One thing that catches my eye here is that the adult-type scapulars have obvious narrow brown sub-terminal fringes. This is something I don't see on kamtschatschensis, which usually has a brown centre line of variable thickness.
Eye-catchingly white below with a narrow tail band and clean base.
November 12 & 21. From a distance the rump and tail are clean white offset with a narrow, sharply defined tail band and a necklace of spots in the longest uppertail coverts.
November 26. For comparison, an "advanced" kamtschatschensis which has already replaced a few mantle and upper scapulars at this early date. It's much browner overall compared to the grey and white presumed heinei above. Though the greater coverts bar invariably looks brownish early in the season the fringes do fade to create a greyer appearance with time.
Those shots appear to confirm the generalisation that kamtschatschensis is a large heavily built "Common" Gull but this can be misleading as there are plenty of small delicate birds with typical kamschatschensis plumage. Interestingly smaller birds seem to show more variation than the more consistent large gulls. They range from light birds with neat tail bands and whiter underwings suggesting heinei or heinei intergrades to much darker individuals that might even recall Mew Gull at first glance.
Below is a group of three typical first winter kamtschatschensis, all with largely grey saddles and faded wing coverts.
Below are flight shots of three rather different but nonetheless typical gulls. One thing they all have in common is a whitish greater coverts bar running across the otherwise very dark underwing.
The striking bird below is more faded than most and the very abraided greater coverts are unusual, it is nevertheless an undoubted kamtschatschensis. Perhaps it's best thought of as transitioning to first summer judging by the completely replaced scapulars, usually there are one or two of the lower rear feathers remaining, outstandingly faded first generation feathers and unusually worn greater coverts revealing more of the secondaries. The folded wing looks better suited to Slaty-backed rather than a Common!
The following bird is the more usual snowy-white worn type, if a small one, with a mix of first and second generation mantle and scapulars and less uniformly faded coverts.
Below are three shots of another small bird, this one nicely marked and with the typical dark underwing and barred rump and uppertail coverts. It has a narrow heinei-like tail band, proof, if any were needed, that single features aren't reliable.
I previously posted on a possible Ring-billed Gull in Osaka and Peter Adriaens pointed out that the median coverts were wrong for RBG. So some of the images are worth repeating here as further "Kamchatka" Gull variation.
To contrast with it are a couple more March birds at the opposite end of the spectrum; small and dark. Had I seen something like this much earlier in the winter, I might have considered the possibility of Mew Gull.
March 23. The black looking tail is remarkable, in fact only the outer webs are black to the base but the inner webs are heavily barred. The solid brown greater coverts across the feather bases is odd at this time of year and the rather uniform greyish back/head and breast is odd at any time.
March 21. Two similar shots showing the head and bill proportions of the bird, the upper with comparison with several typical birds, not what most birders would associate with kamtschatschensis.
March 21. The rump and uppertail coverts are much more heavily marked than normal, almost barred white rather than brown.
March 21. The elongated gull in the foreground is the same bird giving a rather different impression of its structure. Of interest during the week it was present it was almost inseparable from the equally small but very different looking bird on the left.
So to say there's a wide range of appearance within juvenile/first winter Common Gulls in Osaka is an understatement.