Most difficult and thereby very interesting ;-).
This bird, however, does not appear to be a Slaty-backed Gull. Therefore, it would have to show different contrast on the underwing: primaries and secundaries should be pale, contrasting with the darker underwing coverts. For example:
http://www.hkbws.org.hk/BBS/viewthread.php?tid=15932.
In my view, the characters fit a Heuglin's Gull better. The answer can be found here:
http://www.hkbws.org.hk/BBS/viewthread.php?tid=8905.
Cheers,
Bart
