Vega Gull in its first winter can be identified in HK by the following features:
rather pale and uniformly-patterned greater coverts showing no moult contrast;
pale inner primaries visible in flight;
tail band that on average is slightly narrower than heuglini, and also, on average, rather browner;
slightly bulkier and shorter-winged structure than heuglini (not as marked as Slaty-backed in either respect);
late in the winter, relatively darker underparts and somewhat darker base to the bill.
The first two features should be visible on all birds, I believe, if only to rule out the possibility of hybrid origin. Not all the remaining features will necessarily be visible on all birds.
Geoff
