I've been championing Erith Pier for some time now as the potential spot for good looks at a juvenile Caspian Gull this summer and it was great to for the prediction to ring true this weekend. The pier is a productive site, jutting out into the Thames Estuary opposite Rainham landfill site and attracting plenty of gulls, particularly on Saturday afternoons and Sundays when the tip is closed.
I hadn't been at Erith long on the morning of Sunday 28th when I picked up an interesting bird settled on the water. While not a 'classic' the bird evidently showed several Caspian Gull features, looking large and long necked when settled on the water. A couple of slices of bread soon got the bird in the air, revealing a nice, pale underwing and, typically for a Casp, it was vocal and aggressive, giving the distinctive harsh call of the species.
While the bill (quite heavy and stubby looking) and extensively fringed pale greater coverts (and perhaps scapulars too) have attracted criticism from some quarters, I can't personally see anything other than a Caspian Gull here and, actually, when you take into account a bit of moult and wear, I think this bird will look really quite decent. While cachinnans averages a long and slimmer-looking bill than both Yellow-legged and Herring Gulls, this feature is quite variable and birds like this pop up more regularly than most would think (see this one I photographed last year in Norfolk, for example). Of course, there's no way of knowing for sure if this bird has Herring Gull genes in its ancestry, but the vast majority of features range from 'acceptable' to 'good' for a cachinnans and I don't see any reason to label it a hybrid.
Others have asked why this isn't a Yellow-legged Gull. Perhaps the first thing to look at is the head. It's small and rounded, not robust and angular like a michahellis. Facial expression is also important here: it's open-faced, lacking the 'evil' look of a Yellow-legged that is accentuated by the eye mask. I also find the white eyelids of Caspian are much more prominent (although these obviously aren't easy to see once they become white headed) and give the face a distinct look. Bill structure is also slightly different. Yellow-legged has a more angular and 'pointy' bill, often appearing quite bulbous towards the tip. The wing coverts and tertials of Yellow-legged are generally very sharply fringed with pale and uniform on the interior, with very strong notching on the inner coverts (though this can vary, with some having almost none at all). This bird's diffuse markings on the inner greaters is much more Caspian-like. Tail band varies, but less tapering towards the outer tail feathers is better suited to Caspian. Underwing is also variable, with some Yellow-legged being very pale (although this clearly favouring Caspian). Most significant, though, is the call – Caspian sounds very different to Yellow-legged. I posted a photo on Twitter of this bird perched next to a Yellow-legged Gull to illustrate what a different beast it was; you can see that here.
When it comes to juvenile gulls, I think it's best not to over-analyse whether a single feature is acceptable or not. The more and more you look at these birds, the clearer it is that individual aspects of a bird's plumage (especially) and structure are extremely variable and routinely overlap. Judging holistically, assessing behaviour, taking into account what you see and hear is so much more valuable than concluding from a single photo.
The musings of a wildlife enthusiast, usually armed with his camera.