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 9. First-generation tertial pattern. Diffuse white tip like common Gull (score 0).|
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).
|Trait 10. Scapular pattern (second generation). Strong, contrasting shaft-streaks, anchors and/or dark central diamonds on most feathers (score 3).|
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)|
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.