Hybrid Caspian x Herring Gull

[adriaens]

Saw this 1st-cycle gull at Zeebrugge-Heist (Belgium) today:
http://picasaweb.google.be/Zorkyyy/GullSpec#

Quite a short bill with obvious gonydeal angle; rather uniform brown lower hindneck; still many juvenile scapulars left; brown underwings...
Would you agree that this is most likely a hybrid Caspian x Herring Gull ?

[Cristian Mihai]

Why it isn't an odd Yellow-legged Gull?

[lou bertalan]

hi peter,

i consider both plumage (dark underwing, broad tail band) and bill structure well inside variation of caspian. even in combination with late scapular moult although that could indeed point towards some argentatus. but i think it is hazard to guess. either a pure cachi or a hybrid, really hard to say without ring proof. i'm a bit worried about the white corners of tail, formed by white eating into outer rectr.; that's nothing i can remember from a caspian.

lou

[adriaens]

Why it isn't an odd Yellow-legged Gull?

Would be very odd, no ? Its plumage looks unusually fresh for YLG at this time of the year, the bird has retained a lot of juvenile scapulars, there is no dark mask around the eye (only some thin dark streaking), the pattern of the new scapulars fits Caspian better, bill base is turning pale, there are no less than 7 primaries with strikingly pale and contrasting inner web, and tail band looks broader than in typical YLG.

@ Lou: If I had only posted the 4th photo (see below), would you have identified it as a Caspian Gull ?

[JanJ]

First impression of Peters gull standing, reveals a not so fishy Caspian Gull, give and take a possible short-lookning bill, where the gonydial angle dont look so alarmingly pronaunced, however, perhaps a bit long. It´s when we see the underwing coverts/axillaries and the well patterned underparts, belly and the upper breast that it becomes not such a typical Caspian, at least not the looks we have become accustomed to for this species at this time of year, meaning body in this case. Retained amount of juvenile scapulars is not so common for 1cy Caspian at this time of year but I would not make such a big deal about that, since I have seen a few pefectly typicall (from an western autumn point of view), birds at this time of year with a close to, amount of rear juvenile scapulars. Question is, however, how much attention should be payed to the notched edgings, especially to those lower upper ones, if someone should wonder about that. Not unusual for juvenile scapulars in Caspian to marked like this, usually less well-marked.
There´s always some problematics around how heavily marked a 1cy Caspian is allowed to be on it´s underwing coverts and axillaries. Some say (including myself at times) that it´s whitin the variation, some put it down as argentatus influence and so on. More to be learnt here I guess.

From Nov.

http://waarneming.nl/foto/view/969743

and from Sept.

http://waarneming.nl/foto/view/1550433

There are coulntless pic of 1cy Caspian with more or less heavily marked underwing coverts/axillaries, early or later in the autumn.

JanJ

[lou bertalan]

Why it isn't an odd Yellow-legged Gull?

Would be very odd, no ? Its plumage looks unusually fresh for YLG at this time of the year, the bird has retained a lot of juvenile scapulars, there is no dark mask around the eye (only some thin dark streaking), the pattern of the new scapulars fits Caspian better, bill base is turning pale, there are no less than 7 primaries with strikingly pale and contrasting inner web, and tail band looks broader than in typical YLG.

@ Lou: If I had only posted the 4th photo (see below), would you have identified it as a Caspian Gull ?

with that photo only i'd have left it unidentified ... still that underwing pattern resembles more a cachinnans than anything else. it has small sharp arabesque pattern on a rel. pale ground, paler than in many birds i've seen at beginning of sept. in danube delta. actually the large number of dark birds was striking and i guess it's more likely that these where pure caspians than being so many birds from belorussia and russia with apreciable amount of argentatus "blood". look at this e.g. - would you have ID-ed it as a caspian with this pic alone?

IMG_5864 - Kopie.JPG (78.91KiB)Viewed 7330 times

so, darkness is not the problem. dark birds usually have more marked underparts as well, a broader tail band and more spotting on upper- and undertail coverts. also the pronounced head streaking (after postjuvenile moult) is probably connected to the same gene cluster(?) producing well marked birds.
i think upper mandible is a little bit overgrown causing the stronger downcurve and 'longer' and horizontal part in lower mand. between gonys angle and tip.
head and eye placement, and esp. greater coverts look very like in a good caspian. however, with the numerous polish hybrids that hang around in western europe, i think it is not safe to settle such birds as pure cachis. at the same time i wouldn't see such a strong indication for a hybrid like you, given the apreciable number of similarly looking pure caspians at their breeding grounds.

in this order of ideas: are there any results of the polish research team available yet which ringed and studied cachinnans this summer in ukraine?

[JanJ]

Another one. Note the inner hand and well-marked underparts and undewing.

http://www.artportalen.se/birds/gallery ... eID=256295

Jan

[lou bertalan]

this one has a similar bill, otherwise a perfect caspain:

http://i.pbase.com/o2/02/786002/1/12827 ... emsida.jpg

http://i.pbase.com/o2/02/786002/1/12827 ... emsida.jpg

[adriaens]

Thanks everyone for your comments.
I am more used to seeing a distinct whitish bar across the greater and/or median underwing coverts in 1c Caspian Gulls, as in most of the images people have linked to here.
This seems to be missing entirely in the Belgian bird.
Also, I had almost written it off as a (Baltic?) Herring Gull when I had watched it standing for a few minutes. I only started taking photographs because I was not sure.