Dear Nick,
Its a juvenile Heuglin's Gull and the earliest autumn date according to the Avifauna [which quotes the four I saw on 25 September 1977 as the earliest]. I saw the same bird on the scrape at Mai Po last Saturday 19 September between about 1100h and noon.
The main reasons why its not a Slaty-backed are as follows:-
1. The black tail band is not so wide and doesnt cover most of the visible tail. In addition the tail band contrasts with white inner tail and upper tail coverts whereas Slaty-backed would be browner with less contrast.
2. It has dark brown centred wing coverts with only relatively thin pale edges and a few notches to give an overall very dark appearance to the upperwing and upperparts. In particular the greater wing coverts form a dark bar against the blackish secondaries like Lesser-backed Gull, whereas Slaty-backed has paler coverts contrasting with the darker flight feathers.
3. The primaries are mostly all dark whereas Slaty-backed has dark on one outer webs and tips and is much paler on the inner webs forming a distinct paler panel across the inner primaries similar to Herring Gull and Vega Gull.
4. The underwing coverts and underparts have a lot of white and are not so uniform or dark as Slaty-backed.
5. The head has a lot of white whereas at this early stage Slaty-backed would be darker.
6. It doesnt have the dark pink legs of Slaty-backed.
7. Lastly, it lacks the big-headed, heavy chested, broad-winged, rather dumpy short-tailed look of Slaty-backed.
I'm glad you got such good photos to support this unusual early bird.
Mike Chalmers
[ Last edited by cmichaell at 21/09/2009 21:43 ]
