Monday, 24 December 2012

Shoreham Caspian Gull or hybrid (24 December 2012)

24 December.  At Shoreham Fort at high tide this morning a small white headed Herring Gull type was standing on the inner east arm.  More interested at the time in checking for colour rings (none seen) I took a couple of images of this bird but soon lost interest as it didn't do anything, my camera battery was playing up and I had to leave to do some Christmas shopping.  Reviewing images later it looked more interesting than it first appeared - that is the problem with digiscoping - too much messing about and not enough studying what I'm looking at.  Checking against 'Identification of Caspian Gull - part 2' in last December's British Birds (Vol 104 p702-742) I thought it looked sufficiently convincing, with a few minor issues, for a Caspian Gull to try and trait score it.

Here is the bird.  Another that will require a bit more research ...


a small headed snouty gull with long wings, white tipped dark tertials but rather white greater coverts
Trait scoring this bird (as per British Birds, see details below) is a bit subjective but I get 23.  This is in the zone between a pure Caspian Gull (12-25) and a hybrid (22-32).  Pure Herring Gull clocks in at 29-37.  I briefly saw its underwing, which wasn't gleaming white but neither was it very dark.  Its legs do not look particularly long and thin but its front and back ends are convincing.  I'm glad I've seen one before. 

Scapular moult 0/2 (no first generations scapulars left), 92% of Caspian Gulls and 78% of hybrids are thus.

Greater covert pattern 3/3 (lots of white), just 4% of Caspian Gulls but 50% of hybrids are like this.

Ventral bulge 1/1 (despite my best efforts I can't really see one), 38% of Caspian Gulls and 66% of hybrids are like this.

Primary projection 0/3 (long, > 0.6), 72% of Caspian Gulls and 56% of hybrids are thus.

Greater covert moult 5/5 (no moult), 35% of Caspian Gulls and 78% of hybrids are like this.

Median covert moult 5/5 (no moult), 19% of Caspian Gulls and 67% of hybrids are like this.

Tertial moult 3/3 (all old), 66% of Caspian Gulls and 89% of hybrids are like this.

Darkness of head and body 2/4 (light streaking), 43% of Caspian Gulls and 22% of hybrids are like this.

Tertial pattern 0/3 (diffuse white tip), 65% of Caspian Gulls and 33% of hybrids are like this.

Scapular pattern 4/4 (all contrastingly patterned), 15% of Caspian Gulls and 88% of hybrids are like this.
Also at Shorehan Fort were, eventually, all 12 Purple Sandpipers and the Black Redstart while a handful of Brent Geese were flying past, 11E and 8 W).  A low tide visit to the Adur was again disappointing  with no exposed sandbars and a bait digger on the shore.  About 1000 large gulls were right out on the airfield but most were Herring.  Too far to pick out anything interesting or see, let alone read, any colour rings.


Purple Sandpiper on the beach before joining the high tide roost on the wooden inner pier

Black Redstart by Shoreham Harbour's West Arm.  The bird has a rather strange almost rufous tint to the face and upper breast
nice not to be a gull

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