miércoles, 31 de marzo de 2010

Opiniones sobre la supuesta thayeri


Tras el reportaje fotográfico, vayamos con las opiniones que se han ido generando desde la observación de esta gaviota la semana pasada.

-------------------------------------------------------
Martes, 23 de marzo

JanJ
I think we can count kumlieni out on structure, bill size and bill pattern together with the dark tail and primary pattern. As much as I also doubt thayeri, could it anyway be a third cycle type thayeri? 

Theo Muusse
Or, as strange as it may sound, a 3rd winter with a lot of black in the tail, smithsonianus or its counterpart, argentatus? The strange thing for me is the brown colour of the 3rd gen primaries in combination with this dark tail...

JanJ
Strongly pink legs are interesting. A few thayeri showing 2nd and 3rd types cycle plumages (varible). Note that the innner primaries on the subject gull (the one from Spain here) looks adult like which suggest a bird older than 2nd cycle.

Miércoles, 24 de marzo

Martin Reid
I'd say it is definitely NOT kumlieni - too dark in the tail and primaries.  It looks like a good contender for thayeri to me - the only small minority feature that I can see on a quick look is the pale eye - but i don't think this is a blocker for thayeri. Hopefully you'll get feedback from the West Coast of North America where thayeri are common. 
Nice bird!

Theo Muusse
My problem (amongst others) is that the bird shows a 2nd winter like tail with 3rd winter like coverts. The secondaries look rd generation, thus this bird must be a 3rd winter. A 3rd winter with such a tail and brown-marked primarie-patterns is odd or impossible for argentatus. The legs do look the american bubble-gum colour.
Do thayeri-like birds regularly show such a dark tail?

Jueves 25 de marzo

Bruce Mactavish
This is indeed an odd gull.  Not a Kumlien's. Outer primaries and tail band 
too dark. What about the very pale eye especially in a sub adult plumage and all those heavy splotchy marks on the breast and neck?  I've never seen legs and feet that purple on a Kumlien's. The small rounded head and bill 
size look in the Kumlien's/Thayer's group. And the legs couldn't be anything else. The eye is extraordinarily pale for a Thayer's. I am guessing it must be a Thayer's if not some aberrant Herring Gull!

Chris Gibbins
Wow - for sure this is either a kumliens or a thayers.  Its jizz is not correct for glaucous winged and there is no reason to suspect a hybrid herring x Iceland as everything fits kumliens and thayers. The problem is which of the two it is.  It looks like many thayers but also darker examples of kumliens are like this so it is very tricky.  I suspect it is a dark kumliens (and of course this is the most likely geographically) because the flight photos show rather pale brown outer primaries; thayers primaries at this age should normally be a bit darker, but of course in spain you have to consider the possibility of sun bleaching). I am confident to say that it looks like one of these two rather than being anything else. It would be very interesting to know how American experts would rule out thayers as it has the general appearance of one.

Martes 30 de marzo

JanJ
Looking over again at the subject gull, with the new pic. thayeri stands out as the most possible choice. I therefore took the opportunity putting it on ID_FRONTIERS yesterday and both Bill and Alvaro suggested thayeri for the gull.

Alvaro Jaramillo
Unless this bird is really big, I would have no hesitation in calling this a Thayer's if I saw it here! They routinely look like this and there is ample variation in that third cycle age class, some (assuming correctly aged) are more adult like, others more retarded like this. Pale eyes are common in Thayer's Gull, particularly common is this tone of yellow which has brown flecking in it (so not as pure as on a classic 3d cyclesmithsonianus). Structurally this bird looks fine for Thayer's, the blob-tipped look of the bill appears to be an illusion due to the shape of the dark/light on the bill. What color on the orbital?

Bill Rudden
Here a late Dec. Midwest USA 3RD cycle Thayer's Gull that matches your gull.

Martin Reid
Following up of some of the comment/questions about this bird on the
gull-research.org forum:- Aging birds like this is really an educated (or not) guess...I feel that the extent of dark in the tail and outer wing coverts, the non-adult-type outer secondaries, the brown-ness of the primaries with their lack of any white apical spots (instead just the thin crescent of 2nd-generation Ps)add up to a 2nd-cycle bird, i.e. in its 3rd CY. There are a couple of features that are more 3rd-cycle-like - the faint tiny mirror on P10 and the extent of adult-like feathers in the inner upper wing coverts - but IMO these are trumped by the
predominance of 2nd-cycle features. As a 2nd-cycle Thayer's it looks perfectly normal to me except for the pale eyes - but Alvaro has already addressed that as being fine for this age-class of Thayer's. Yes, this age of Thayer's typically does have extensive dark in the tail and outer primaries. Al asked about size; the original photos show it with YLGUs and it clearly is not too large for Thayer's. Of course, just because Alvaro and I would call it a Thayer's here in America does not make it one - but it does establish that, if no other reasonable ID from the Old World is forthcoming, it is perfectly fine for Thayer's and from a New World point of view no other taxon or hybrid is a better fit (Alvaro please correct me if I'm wrong on this). So can you Euro-gullers reasonably rule out anything from your world? As an erstwhile Euro-guller myself, albeit with rather limited recent experience, I don't see any good pure fit and think we'd have to strain quite hard to conjure up a hybrid scenario for this individual,after taking into account all its features.

JanJ
I had noticed your comment on Birdinggulls Yahoo Group Martin, which gave me some feedback as to my own suspicion about this gull. Regarding the possible ageing I agree with Martin - although this can be a challenge due to possible retardation or advanced plumage features i imagen. As can be seen from the pic. Alvaro, the size can be fairly judged
alongside the michahellis nearby, which as Martin mentioned, looks fine sizewise for thayeri. At the moment, I fail to see any features that would suggest argentatus/argenteus, (counting smithsonianus in?), even the odd one (I dare say).

Alvaro Jaramillo
I am sure that ageing gulls is quite reliable, but I wonder how many baseline studies or online photos there are out there of known age birds? It seems to me that until we get a good deal of known age birds to learn from, this will be at least a certain part guesswork. In effect we don't know how to assess how much of the time we are wrong when ageing gulls! It will also
be interesting to learn how different species age, as it is certainly not uniform even within gull size classes.


Miércoles 31 de marzo

Mars Muusse
Theo is right in the fact that the inner secondaries are pretty clean grey based and would indicate 3rd gen feathers over 2nd gen. But the pattern of brown centres on the tertials & the bleached brown primaries with faded tips could well point to a bird one year younger (3cy March by now). Of course, many example birds with mirrors on 2nd gen P10 have came accross in other topics and may still apply to several taxa. Maybe the pattern on inner secondaries can be explained by the time of replacement (which is normally late in the sequence)? In general, the bird shouts 3cy imo.
--------------------------------------------------------------------------------

Iré añadiendo actualizaciones a esta entrada, según vayan surgiendo comentarios nuevos. Por el momento, sirva de resumen decir que es necesario determinar con exactitud la edad de esta gaviota, ya que parece claro que se descartan especies de origen europeo, y nos queda por tanto la duda entre thayeri y kumlieni.

No hay comentarios: