Circumstances of observation. As reported on my blog, Frank Lambert and I found a first-winter (2CY) Caspian Gull about half-way along the West Arm of Newhaven Harbour on Sunday 5 February. It was present from at least 10.15-11.10am. Although it was rather distant, and the light poor for photography, the digiscoped images copied below show most of the necessary features. It was also seen briefly in flight, by both of us, and showed a very pale whitish underwing, white rump and solid black tail bar. John Cooper saw the same bird the following day when it, or a bird appearing similar, was also seen by Tony Cook, Matt Eade and Paul James.
|Newhaven Caspian Gull when first picked out|
Trait scoring. Several highly competent observers have expressed no concerns at all with this birds identification, considering it a classic, but would it stand up to the latest criteria – trait scoring? Identification of Caspian Gull – part 2 by Gibbins, Neubauer & Small (British Birds 104:702-742) is the essential reference for this analysis.
For first-winters seen between October and March the above paper provides a system of scoring based on certain ‘traits’ to differentiate most pure Caspian Gulls from Hybrids. When added up these trait scores will indicate whether a bird is a Caspian Gull (combined score 12-25), Hybrid (22-32) or Herring Gull (29-37). Birds in the overlap zone are best left indeterminate. For a safe Caspian Gull identification a score below 22 is required although the lower the score the better.
Trait 1. Extent of scapular moult. On this bird no first-generation feathers are present. This feature is exhibited in 92% of Caspian Gulls but only 13% of Herring Gulls and 78% of hybrids (based on samples of 63, 85 and 12 respectively). Score = 0.
Newhaven Caspian Gull showing all second generation scapulars.
Trait 2. Greater-covert pattern. This bird had white edges with delicate notches or vermiculations or dark brown centre with white tip to 1/3rd of length. This is shown by 63% of Caspian Gulls, 0% of Herring Gulls and 12% of Hybrids. Score = 1.
Newhaven Caspian Gull showing delicate greater-covert pattern. This also shows the very long, thin legs. Not a trait feature for first-winters but it is for adults where it would score 0.
Trait 3. Ventral bulge. This bird showed a ventral bulge, as did 62% of Caspian Gulls, 14% of Herring Gulls and 34% of Hybrids. Score = 0.
Newhaven Caspian Gull showing ventral bulge
Trait 4. Primary projection. This bird showed a very long primary projection (>0.6). Consistent with 72% of Caspian Gulls but only 6% of Herring Gulls and 56% of Hybrids. Score = 0.
Trait 5. Greater-covert moult. Scored from 0-5 depending on the extent of moult (0 almost all, 5 none). Images of this bird and my ability to recognise replaced feathers make this very hard to assess. I can perhaps excuse my incompetence in this area as by early February all second generation feathers would be 4-6 months old and so subject to some wear. It appears that none have been replaced which would be consistent for 35% of Caspian Gulls, all Herring and 78% of Hybrids and would give a score of 5.
|Newhaven Caspian Gull appearing to show all first generation greater coverts|
Trait 6. Median-covert moult. Scored from 0-5 depending on the extent of moult (0 almost all, 5 none). As above, images of this bird and my ability to recognise replaced feathers make this very hard to assess, more so as second generation feathers would be 4-6 months old, but juvenile median coverts would be darker with more contrasting centres so many may have been replaced. Assuming the worst, that none have been replaced, would be consistent for 19% of Caspian Gulls, 99% of Herring and 67% of Hybrids. This would give a score of 5 although its true score is probably lower?
Trait 7. Tertial moult. Scored from 0-3 depending on the extent of moult (0 three or more new tertials, 3 none). Hard to judge but all tertials are assumed to be old. This is consistent with 66% of Caspian Gulls, 100% of Herring Gulls and 89% of Hybrids. Score = 3.
Newhaven Caspian Gull showing what appear to be old tertials. Also evident the dark tail bar and impressions of the sides of a white rump.
Trait 8. Darkness of body and head. This bird has reduced greyish wash or streaking confined to flanks and single streaks around the neck. 41% of Caspian Gulls show this but none of the sampled Herring Gulls or Hybrids. Score = 1.
Newhaven Caspian Gull showing reduced streaking confined to flanks and nape, although a score of 0 is almost deserved in this image. Also long legs, small head, predominantly all dark bill and protruding breast are noticeable in this image.
Trait 9. First generation tertial pattern. Shows a diffuse white tip like a Common Gull. Consistent with 65% of Caspian Gulls, 1% of Herring Gulls and 33% of Hybrids. Score = 0.
Newhaven Caspian Gull showing thumb-nail white tips to tertials
Trait 10. Scapular pattern (second generation). Shows strong, contrasting shaft streaks on less than half of feathers. Consistent with 30% of Caspian Gulls but none of sampled Herring or Hybrids. Score = 2.
Newhaven Caspian Gull showing contrasting shaft streaks on less than half scapulars with the odd diamond-shaped dark centre just about visible in this image
Trait summary. The above trait scores, assuming the worst for covert moult (traits 5 & 6), total 17. An average median-covert score reduces it to 14. The average overall trait score for first-winter Caspian Gulls in the sample was 18.2, within the range 12-25. The range for Hybrids is 22-32. This bird, even with the worst median-covert score has a trait score below the average for the species putting it towards the more distinctive end of the range. This is no surprise given its classic appearance.
|Newhaven Caspian Gull - the complete package.|
Non-traitable features. Other features noted that are not 'traitable' were the whitish underwing, white rump and dark tail band (seen by both Frank and myself in a brief flight view but sadly not documented), an almost completely dark bill, bill shape (L:D ration of about 3, trait score = 0 were it an adult), small head with small forward placed eye and bulging upper breast. Most of these features are evident in the following images.
Caspian Gull at Newhaven showing a suite of classic features, in particular its very long thin stilt-like legs, protruding breast, small head and forward placed eye
Caspian Gull at Newhaven showing a suite of classic features, in particular its small head, forward placed eye, lack of appreciable eye mask and in this view a bulging upper breast and ventral bulge
|Newhaven Caspian Gull showing a suite of classic features, this angle showing its attenuated rear end|