cmichaell

The bird in your photo is an adult Heuglin's Gull. Apart from its yellow legs, it is Heuglin's because its mantle and upperwings are relatively dark grey with little contrast between the grey and the black primary tips compared to other local gull species. It also has a small white mirrior on its outer primary [p10] which is typical for Heuglin's whereas most other large gulls have larger mirrors and usually on both the outer two primaries. Another good feature at this time of year is primary moult. You can see this bird has four old outer primaries [p7 to p10] with new primaries growing at p5 and p6 [you can see the black tips with white spots not yet full length]. This is typical for Heuglins in November, as many do not complete their wing moult until well into the new year whereas most other large gulls in Hong Kong have either completed wing moult or are near completion by now.

The english name "Yellow-legged Gull" is confusing here as its main representative is the taxon mongolicus which normally has pale pink legs in Hong Kong and is better referred to as Mongolian Gull.

Hope that helps.

Mike Chalmers

cmichaell

Geoff,

Agreed. In this case it is hard to count how many new inner primaries are present as it is difficult to discern the individual feathers in the photo. However, the middle primaries can be identified by the extent of black on the tips. Heuglin's Gulls normally have the black across both inner and outer webs of the ends of the six outermost primaries from p10 to p5, with a final small black spot usually present on the outer part of p4 in fresh plumage but not across the full width of the feather.

In the photo, p4 with its small black spot appears fully grown, the adjacent p5 is 1/2 to 2/3 grown and, as you point out, the new p6 is not visible because it has only just started growing.

Whilst primary moult varies between individuals, it is a useful identification tool in early winter. In Hong Kong, Heuglin's Gulls have the latest primary moult, and for a large gull to have a moult score of only about half maximum by the end of November indicates it can only be Heuglin's [or a hybrid with some Heuglin's genes].

It would be useful if photographers could try and get photos of all adult large gulls this winter, either in flight or wing stretching, so that moult differences can be compared.

Mike

cmichaell

Head streaking on winter adult large gulls varies a lot and is more of an indicator than a diagnostic feature.

In general, winter plumage Heuglin's gulls have relatively fine head streaking usually strongest and most concentrated in a "shawl" across the rear neck. Yellow-legged Gulls often appear to have no head streaking, but when you get close-up may have some very fine streaking, especially on rear neck, and Vega and Slaty-backed usually have the strongest head streaking, although there is wide variation and some Vega can look similar to Heuglin's. Remember also that this feature only works in early to mid winter. By late winter/early spring, adults start to assume breeding plumage and then they all have unstreaked white heads!

Mike