But just what do they really look like?
Juvenile Caspian Gulls (and indeed Yellow-legged Gulls) are already independent and on the move. A key trait of both Caspian and Yellow-Legged Gulls is that juveniles disperse rapidly away from their natal colony and, in many cases, they roam widely. So we can expect and should be on the lookout for them here in Britain, even now in mid July.
The problem is that juveniles are very different to the first winter birds we are used to seeing over the winter months. Due to a combination of moult, wear and fading, first winter birds are generally rather striking creatures. In July and August, juveniles are different, being crisp and fresh, with no (or extremely limited) moult and they generally look rather dark on the head and body – quite unlike the image we have of this species. I’ve just come back from Azerbaijan, where our objective was to learn more about Caspian Gulls (I was with Visa Rauste and Hannu Koskinen, friends and fellow gull enthusiasts from Finland). Being on the Caspian Sea, Azerbaijan is ideal for such studies, as it is far from the hybrid zone that now exists in western Europe; we can therefore be sure that the Caspian Gulls in Azerbaijan do not have any Herring or Yellow-legged Gull genes, as many of the birds in Europe undoubtedly do. So just what do these ‘real’ juvenile Casps look like?
This post is meant simply to illustrate what birders searching for juvenile ‘Casps’ should be looking for at this time of year – it is not analytical, merely a photo record that I hope is a useful reference point (few images of juveniles from the Caspian heartland of the species’ range have ever been published). The main thing that should be evident from the selection of images that I’ve chosen is that they are very variable, in terms of both structure and plumage, and many are rather dark.
Differences between the individuals featured below are evident both on the ground and in flight. On the ground notice that some have classic Caspian jizz, but others do not –in fact resemble Lesser Black-backed Gulls. On the ground also notice differences in the greater covert patterns between these individuals, and also the tertials. More especially, notice how dark and well streaked some are on the head and body. Juvenile Casps are not white. In flight, notice differences in the pattern on the inner primaries – some darker birds are very like Yellow-legged Gulls while paler individuals have silvery inner webs to the inner 5 or so primaries, with pale patches, stippling and a darker feather tip. Most importantly of all, note that, contrary to more or less everything that is published, they can have well marked (and hence dark looking) underwings at this age.
So, if you encounter an odd looking juvenile gull in the coming weeks, don’t write off Caspian just because it is dark/well streaked and/or does not have the pure white underwings you were expecting – real Caspian Gulls from the Caspian can be dark. Such features are not necessarily a sign that you have a hybrid. Expect the unexpected.
This final bird shows classic structure and plumage; simply remember that not all are like this.