Glaucous Gull - Adult or 4th winter(5cy)?

[richard lowe]

This adult-type Glaucous Gull was present at Albert Village Lake, Leicestershire, England on 21st January 2016. There are no visible immature feathers but the bill is pink-tinged with a dark subterminal mark. Is this an 'adult' with sub-adult-type bill or a 4th winter with adult-type plumage? (See attached video-grabs.)
Any opinions welcome.
Kind regards to all, Richard Lowe (Derbyshire, England.)

[marsmuusse]

Hi Richard,
With a completely white plumage, including underwing coverts and white tail, and no black streaks in the primary coverts it is only the bill that stands out as possible immature. However, even very old adults may still show such black markings.
An example bird could be 18CY (!) blue BZAB:
http://www.gull-research.org/hg/hg5cy/bzab.html

Still, of course I can not exclude your bird indeed is a 5CY aged adult.

[richard lowe]

Many thanks Mars - concise and informative as ever.
Regards, Richard.