msamuel

By looking at the two bigger gulls' eyes, I think they are two different species.  Is the right one with a sharper yellowish eye a Slaty-backed gull and the left one a Heuglin's gull?  Thanks in advance.
The photo was taken on 15Mar09 at Mai Po.
睇兩隻大鷗的眼神，好像兩個不同品種，右手面的是否灰背鷗，左手面的是否烏灰銀鷗？
相片於零九年三月十五日在米埔拍攝。謝謝！


[ Last edited by msamuel at 12/07/2009 18:53 ]

cmichaell

The gull on the right is definitely not Slaty-backed because it doesnt have a dark-enough mantle and doesn't have dark pink legs. It also lacks the heavily built structure of Slaty-backed.

Both are adults with yellow legs and both appear to be in the Heuglin's/Vega complex. The difference in iris colour is not diagnostic as it can vary from all yellow, as the right hand bird, through intermediate, like the left hand bird, to all dark in breeding plumage, especially in Vega but also in a small proportion of Heuglin's.

The left hand bird appears to be typical heuglini taimyrensis, the commonest large white-headed gull in Hong Kong. Its mantle is not dark enough for nominate heuglini heuglini. It has faint small spotting on the nape which, as expected by March, is all that is left of the more extensive head and neck streaking characteristic of winter plumage. It has completed wing moult and has a medium sized white mirror on the underside of the closed outer primary. It has a dark mark on the upper mandible above the red spot, but again such markings do occur in several taxa and are not diagnostic. Sometimes they indicate a bird which is just reaching maturity.

The right hand bird is similar in many respects, including the faint spotting remaining on the nape, but differs by having a slightly paler mantle, broader white tertial crescent, larger white mirror on the underside of the closed outer primaries, and no dark markings on the upper mandible. The first three features taken together indicate that it probably originates further east towards the zone of vegae birulai, but because it still clearly has yellow and not pink or intermediate apricot legs it is not birulai. This is where gull taxonomics get messy. The current view is that  both taimyrensis and birulai represent hybrids between Heuglin's Gull and Vegae Gull, and a range of intermediates can therefore occur. Some mongolicus [Mongolian Gulls] can also be very similar to the right hand bird; indeed some authorities now consider mongolicus to be a sub-species of vegae. However, most mongolicus in Hong Kong have pink legs, heavier build, paler mantle, and white head and neck, so on balance I think your right hand bird is still heuglini taimyrensis but closer to birulai than the left hand bird.

The high quality photos such as yours and others, especially from close-ups at Nam Sang Wai last winter, all help to document and compare these confusing large gulls. Keep taking the photos and keep posting them for discussion.

Mike Chalmers

msamuel

Mike, thank you for your impressive educational response.  I did overlook the color of the right gull's legs.
With your encouragement, I included another photo taken on the same day at the same place below.  The right gull has pink legs and is obviously larger (heavily built?) than the gull on the far left, which I think is a Heuglin’s Gull.  It appears to me that it has not fully grown into an adult yet as it still has a black tail bar.  Its paler mantle makes me rule out the possiblity of being a Slaty-backed Gull or a Heuglin’s Gull.  To me, it is a choice between Yellow-legged Gull and Vega Gull.  By the size, I lean towards the former, but the extensive head and neck streaking make it does not look right to me.  Could you or other experts please comment again?

cmichaell

Samuel,

The bird in your new pics is very striking and interesting.

First the easy bit on ageing. The overall grey mantle and white underparts indicate near adult and the broad dark band across the bill combined with the lack of white tips to the primaries and the black still present in the tertials and tail indicate it is third summer. The fine brown markings on the grey wing coverts also indicate sub-adult.

The most striking features are its large size, its aggressive expression and head shape accentuated by the large pale yellow eye set near the front of the head and the heavy bill, and the very pale pink legs. Mantle colour is clearly paler than Heuglins but that doesnt help too much because its still OK for Vega or Mongolian [Yellow-legged]. A typical Vega would however have darker pink legs and often a darker iris. Leg colour is OK for Mongolian gull seen in HK but this would usually have a less fierce expression caused by a more rounded head, smaller eye set more centrally on the side of the head, and often darker iris.  However, there is third possibility which is not yet on the HK List and that is American Herring Gull. The large pale yellow eye, large bill and sloping forehead which combine to give that fierce expression fit American Herring Gull nicely. The dark-centred tertial also fits American Herring. However it should have the palest mantle of the all the possible species, whereas it looks too dark in the photo but this is hard to judge and can vary with light conditions. If you see the tail band, its width would help as American Herring has the broadest band, Vega is intermediate and Mongolian has a narrow sharply defined band. Any flight shots would be a great help as they would also show the wing tip pattern. If the bird had still been in winter plumage, then Vega and American Herring both have very strongly brown streaked heads and necks, but there is no sign of such markings. The faint spotting on the neck is OK for Mongolian.

Thus, your bird remains a mystery. It could just be a pale eyed fierce looking Mongolian Gull but I cant exclude the other possibilities.

Mike Chalmers

msamuel

Mike, thanks again for your detail input. Gull identification is really a challenge.  I heard some of the bird watching friends have already given it up partially if not completely.  Not sure from where else I can gain more knowledge on this part!

Samuel

[ Last edited by msamuel at 30/07/2009 20:32 ]