Juvenile and first winter taimyrensis are distinctive gulls. Juvenile plumage is superficially closer to smithsonianus than anything else though the rump and uppertail coverts and the vent and undertail coverts are much less heavily marked. Within Kansai smithsonianus isn't likely to present any problems and Vega Gull is the taxon in most need of elimination.
AppearanceIt averages smaller than Vega but there's a lot of overlap, not least because Vega is so variable. It's longer-winged in flight and more attenuated at rest. In general taimyrensis is a more uniform, browner gull. In brief the more obvious plumage differences from Vega are:-
Saddle dark centred feathers with fairly even pale fringes rather than strongly patterned type.
Tertials mainly dark with usually narrow fringes on the distal half, sometimes with slightly lighter enclosed sub-terminal area. Vega with more complete fringes and stronger patterning.
Greater coverts with obvious brown bar which may be restricted to the bases of the outer half or more rarely throughout the full length. The innermost are also dark centred with narrow pale fringes much like the tertials. Vega is effectively chequered throughout.
Rump and uppertail coverts are rather white contrasting with the tail. The edge of the rump and longest uppertail coverts have a large dark arrowhead creating a horseshoe of prominent spots but with light markings across the rump. Vega usually more evenly and heavily barred across the rump recalling smithsonianus.
Vent and undertail coverts basically mirror the upperparts with a ring of promintent spots but clean centre. Again Vega is more barred, especially on the central undertail coverts.
When I start gulling at the beginning of November taimyrensis is already present, so I'm uncertain of their arrival dates. They're quite numerous throughout the month, sometimes in discrete flocks of up to 30 and can make up 50% of the large gull population at that time. Numbers drop off in December and by mid-month there might be no more than 10 present. Birds are in full juvenile plumage when they arrive and by the time the last birds are leaving they will often have replaced the mantle a sometimes a small number of scapulars.
All the birds above are typical juveniles and perhaps show a surprising degree of consistency given how variable the adults tend to be. Below are four shots of a bird which does differ from the norm, though it's a matter of colour saturation. The scapulars are darker brown and the terials blackish-brown. All these feathers have very narrow even fringes. The coverts bar is very dark because of a lack of contrast with the gound colour. The rump is more heavily marked than normal.
Below is strikingly different, the only example I've seen of a grey bird and perhaps it is more at the heuglini end of the spectrum.
In December birds begin moult to first winter by replacing mantle feathers and sometimes a limited number of scapulars.
First winter 2CYBirds start to return from about mid-March and the juveniles have transformed into very different but equally distinctive gulls. The saddle has been replaced and moult seems to be ongoing as the most recently replaced feathers, some upper tertials and rear scapulars, inner greater coverts and central median coverts are similar in appearance but darker, not yet as faded as the saddle.
The following images are members of a flock I first saw on 20 March 2012 which was still present until at least 1 April, my final gulling trip that winter.
Finally six digiscoped shots of a distant and very aggressive bird that stayed out at the river mouth. Initially I thought it might be a Caspian based on structure but once I saw the plumage details I realised that wasn't possible. Nevertheless, it's interesting that could be the first thought and again is more suggestive of heuglini.