Wednesday, 29 October 2014

Trait scoring an Adur Caspian Gull

On 5 October 2014 while checking through the gulls on the River Adur at low tide I saw a green colour-ringed gull that I took to be a fairly classic first-winter Caspian.  The CR-birding web-site indicated that it had been ringed in the Lausitz area of East Germany, near Polish border, in a mixed colony where Caspian Gull dominate although hybrids are possible. 

Disappointingly submission of the ring details have as yet, over 3 weeks later, produced no response so a trait score seems the best way of determining whether it might have some hybrid influence (not that I could detect any from my very limited experience of the species). The ground-breaking reference Identification of Caspian Gull. Part 2: phenotypic variaility and the field characteristics of hybrids by Chris Gibbins, Grzegorz Neubauer and Brian J Small was published in December 2011 in British Birds (104 :702-742). Trait scoring can be applied to first-winters between October and March, so no excuse for me not to try.

Scores are applied to ten characteristics for first-winters with pure Caspian Gulls (of known parentage) scoring 12-25, known hybrids 22-32 and Herring Gulls 29-37.  The lower the scores the better with an overall score of under 22 almost certainly ruling out a hybrid.  Here goes:

Trait 1.  Extent of scapular moult.  This bird has no first generation scapulars remaining (score 0).
Trait 2.  Greater-covert pattern.  Clear white notches/barring creating a delicate piano key pattern along the whole edge/feather but much of feather dark (score 2).
Trait 3.  Ventral bulge. Yes (score 0).
Trait 4.  Primary projection. Ratio measures as 0.63 (score 0, > 0.6).
Trait 5.  Greater-covert moult. About half of the greater-coverts are new (score 2).

Trait 6.  Median-covert moult. About one-third of the median-coverts are new (score 3).
Trait 7.  Tertial moult.  All appear to be old (score 4).
Trait 8. Darkness of head and body.  Light streaking/wash to head (incl. some dark around the eye); isolated streaks/blotches on body' (score 2).

Trait 9.  First-generation tertial pattern.  Diffuse white tip like common Gull (score 0).

Trait 10. Scapular pattern (second generation).  Strong, contrasting shaft-streaks, anchors and/or dark central diamonds on most feathers (score 3).
The total trait score calculated above is 16.  This places the bird firmly in the range of pure Caspian Gull (12-25) and well outside that of a hybrid (22-32).

This bird also ticks the boxes for the other features not included in the trait score but that I'd hope to see on a potential Caspian Gull:
long thin legs
long slim bill with slight gonydeal angle (ratio of 2.5 and would score 1 if an adult)
pale underwing
I've waited a long time to see a Caspian Gull on the Adur that I was really happy with.  This very smart looking bird ticked all the boxes for me and I had the added bonus of Paul and Briget James seeing it before it flew off too.

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