Heuglini 2cy, February 28 2001, Bahrain: Manama harbour. 

Powerful bird. All visible feathers in the upper parts moulted to second generation, except outer lesser and greater coverts. Very pale new tertials with a double anchor pattern. Note the pale pink base of the bill. Bird in front: slender-billed gull.

In Bahrain, such large birds with almost parallel bills are either cachinnans or heuglini. Cachinnans, being a southern breeder from the steppe belt, normally doesn't show such active post-juvenile moult in February. Moreover, most cachinnans have finished their post-juvenile moult by early November as 1cy birds. Hence, active post-juvenile moult in February in 2cy birds is a strong indicator for nortern breeders from the tundra region, i.e. heuglini and fuscus in Bahrain.

By July, when some of these birds return to N Europe, the wing-coverts will appear worn and they show just a simple arrow-pattern.