Monday, 3 November 2014

When is a Casp not a Casp?

Last Monday, October 27th, I visited the gull roost for the first time in a while.  It was nice to actually have a pre-roost to look through again and the mown spit gives them areas to congregate on.

One of the first birds I looked at immediately jumped out as a 1st winter Caspian Gull.  Its plumage and structure looked spot on, although in the dim light, I was a bit concerned about its rather too streaky looking head and why it had a weirdly long and mainly pink bill.  However, with everything else looking good, I put this down to variation.  An adult Yellow-legged Gull joined it and 5 adult GBB Gulls were the other birds of note.

The following day, I went down  mid afternoon in the hope of seeing the previous day's birds again.  Fortunately, the 1st winter Caspian Gull was one of the few LWHG present.  In much better light, I was able to study it in more detail and take some photos.  Its provenance was beginning to worry me a little, because although pretty much all of its plumage and structure looked great for Caspian, why did it have such a streaky looking head, particularly flaring out behind the eye? and why were its undertail coverts fairly heavily marked? and what a weird looking bill that didn't seem to fit anything.  Take a look at these:

Adult YLG behind
So, I was left in two minds, it was either a good Caspian, but a slightly aberrant one - have you ever seen a 1st winter LWHG with a bill like this? Or, it was a Caspian Gull with some hybrid genes, that might explain the streaky head and undertail coverts.  If it was a hybrid, the genes haven't affected the mantle, coverts, tertials or underwing, which all look good for Casp.  Structurally it looks good as well, though apart from the features mentioned, I did wonder if the legs were a bit short - nice pale pink colour though.

That's the second Casp this year that has had the spectre of hybrid genes - just waiting for an unequivocal one now for the year list!

At least this was straight forward!

1 comment:

  1. Gull bill colour is variable and if that had a solid black bill I don't think you'd be so unsure. Seems more likely a Caspian than anything else!