Wednesday, 6 February 2013

one and a half Casps

I was on the tip on Saturday, rudely interrupted by news of that big brute of a finch. It didn't however dent my enjoyment of the gulls and after seeing a 2nd-winter Caspian Gull (the same bird that has been regular at Wat Tyler CP recently), the other Caspian-esque Gull was this individual, which I firmly believe comes from that hybrid swarm in/near Poland where cachinnans mixes with argentatus Herring Gulls. So for now I'm going to call this near adult (presumably 5cy) bird a hybrid.
Note the Caspian Gull like stance and structure, clean head, spindly tepid-coloured legs and slim parallel-sided bill. Chris Gibbins' landmark BB paper however states 'a clearly pale eye in a near-adult gull is not good for cachinnans but a dark eye does not automatically rule out Herring Gull'.

Nice remnant necklace. Note the Caspian Gull-like white tip to P10, also shared by argentatus Herring. The pale tongue on the underside of this feather looks atypically dark, while P9 is within range for Caspian Gull showing a banded white tip.
Note the extensive dark retained in the primary coverts. Although the grey tongues bleeding into the black on P7-P10 are obvious here, is there too much black in the wing or is this due to immaturity? Anyway, there is just a delicate and indistinct black line on P5 - too feint for a pure cachinnans?
Interestingly, to throw even more into the mix, a Polish-ringed bird that was seen as a first-winter several winters ago - where it looked like a decent 1st-winter Caspian Gull - was seen again on the tip as a near adult, where it looked decidedly like a Herring Gull. Interesting stuff, and it leaves one to wonder what all these 1st-winter Casps we see in Britain will actually mature into. The couple of adult Med Gulls whacking about were slightly easier to identify!


  1. Hi Rich, interesting bird - very Caspian-like but for P5. I had missed the importance of CG's comment on iris colour, and will have to re-read the paper as I'm not sure why this would be the case - given that ad Caspian is said sometimes to have a pale iris and large gulls often develop pale irides from their second winter, it's not obvious to me why this would be a problem. For reference I recall pics of a spring ad Caspian Gull with a very pale iris on Steve Arlow's website - wouldn't such a bird surely have developed its eye 'paleness' quite some time before adulthood? Plenty of food for thought, as always with gulls!

  2. With a bit of luck, one day all these large gulls will have hybridised so much that they converge into one species. That is, of course, the view of a larophobe!