Big thanx to all who commented on the photo of the gull a few days ago, I also got comments from 3 keen gullers in other parts of Britain via email & PM (thanx John and Rob).
In a nutshell I agree with what’s been said:
It has several pro-Casp features, in particular the underside of the outermost primary (P10) which can clearly be seen to be virtually all-white. The bill shape and colour looks good. The mantle shade, size, jizz, posture, bulging throat beneath the bill etc., all good. It shows the bulging hind-belly often cited as a good feature for Casp. The legs are intermediate, and the eye is pale, though these days it’s considered OK for Casps to show this. One very experienced guller said via email, from this one photo it looks like a Caspo. He then makes the point that it’s not possible to be 100% certain from this one shot, something which several folk said. And i agree – in fact in the field I felt that it didn’t have the right head shape for a Caspo (although you can’t really see this in the pic), and that it prob wasn’t one, so when I reported my news to Birdline (2 Iceland & 5 YLG) I explained that I had a bird with some features of Casp, but wondered about the possibility of some YLG genes in there, as they’re known to hybridise around the Black Sea – there were some interesting photos on a forum of probable hybrids which were taken in Romania a year or two ago. There doesn’t appear to be a consensus on how much variation there is within cachinnans, some argue that a Casp should display all the features of a “classic”, but I would argue that these birds are in fact, extremes. How many Herring Gulls would get accepted if they had to show all the features of a “classic”? And it’s fair to say that the criteria for ID-ing Casps have changed over the years – I recall one of the first papers suggesting that a “good” Casp always has a dark eye, which led me to bin a perfectly good one at the same spot late 90’s – this is no longer seen as a critical feature. Time will tell how much further the goalposts move!
The other side of the argument is that out of range gulls are said to often display “non-classic” features more often than birds from within the core of their range, perhaps suggesting signs of hybridisation where they meet other taxa.
Ultimately I’m still amazed that as recently as 20 years ago this species was a mega rarity, then started to be ID’d more often in SE England, records spreading NW each winter til now it’s seen as a rare but regular visitor to not far beyond our North Wales border! For a bird that’s migrated many many hundreds of miles, is it too much to ask that it continues another 30??
And of course, we should never forget that a photo can make a bird look more like a Caspian than it actually does in the field – or less like a Caspian than it does in the field!
And now that it’s Spring… it’s a good time for white-wingers and Ring Billed!… and don’t worry it’s only 3 months til the YLGs start moving again. ;-)