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| Coordinator: Greg Neubauer Marcin Przymencki Albert de Jong Mars Muusse cachinnans plumages |
From the Rarities Committee's files - Identification of Caspian Gull. Part 2: phenotypic variability and the field characteristics of hybrids, by Chris Gibbins, Grzegorz Neubauer and Brian Small, IN: BB 104/2011. Table 2. Percentages of sample first-winter birds with trait scores for selected plumage, structural and moult characteristics.
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| Trait | Score | Description | Herring (n-85) |
Caspian (n=63) |
Hybrid (n=12) |
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| Extent of scapular moult | 0 | no first-generation feathers remaining | 13 | 92 | 78 | |
| 1 | a small number (<1/3) of first-generation feathers remaining | 43 | 8 | 22 | ||
| 2 | a significant number (>1/3) of first-generation feathers remaining | 44 | 0 | 0 | ||
| Greater-covert pattern | 0 | simple pattern with brown centres and sharp white edges, with no white vermiculation or notching | 0 | 3 | 0 | |
| 1 | white edges with delicate notches or vermiculation; or dark brown centre with white tip to 1/3 of length (i.e. white restricted to tip or distal third) | 0 | 63 | 12 | ||
| 2 | clear white notches / barring creating a delicate 'piano key' pattern along the whole edge / feather; but much of feather dark | 6 | 30 | 38 | ||
| 3 | lots of white (more than 1/2 of coverts looking white) distributed along the whole feather, or a bold notching ('piano key' pattern) | 94 | 4 | 50 | ||
| Bill shape | 0 | very long and slim, with little/no visible gonydeal angle (L:D ratio >2.8) | 0 | 11 | 0 | |
| 1 | slim, slight gonydeal angle (ratio 2.4-2.79) | 3 | 67 | 37 | ||
| 2 | intermediate (ratio 2.0-2.39) | 91 | 20 | 63 | ||
| 3 | short and deep, well-marked gonydeal angle (ratio <2.0) | 6 | 2 | 0 | ||
| Leg length | 0 | long | 0 | 14 | 12 | |
| 1 | moderately long | 13 | 46 | 50 | ||
| 2 | short | 87 | 40 | 38 | ||
| Ventral bulge | 0 | present | 14 | 62 | 34 | |
| 1 | absent | 86 | 38 | 66 | ||
| Primary projection | 0 | very long (ratio >0.6) | 6 | 72 | 56 | |
| 1 | moderately long (ratio 0.5-0.59) | 38 | 25 | 33 | ||
| 2 | medium (ratio 0.4-0.49) | 45 | 3 | 11 | ||
| 3 | short (ratio <0.4) | 11 | 0 | 0 | ||
| Moult: greater coverts | 0 | all or almost all new (>75%) | 0 | 0 | 11 | |
| 1 | 51-75% new | 0 | 3 | 0 | ||
| 2 | 34-50% new | 0 | 2 | 0 | ||
| 3 | 10-33% new | 0 | 24 | 0 | ||
| 4 | one or two feathers moulted | 0 | 36 | 11 | ||
| 5 | no moult | 100 | 35 | 78 | ||
| Moult: median coverts | 0 | all or almost all new (>75%) | 0 | 0 | 0 | |
| 1 | 51-75% new | 0 | 13 | 0 | ||
| 2 | 34-50% new | 0 | 20 | 11 | ||
| 3 | 10-33% new | 0 | 35 | 0 | ||
| 4 | one or two feathers moulted | 1 | 13 | 22 | ||
| 5 | no moult | 99 | 19 | 67 | ||
| Moult: tertials | 0 | 3 or more new | 0 | 10 | 11 | |
| 1 | 2 new | 0 | 8 | 0 | ||
| 2 | 1 new | 0 | 16 | 0 | ||
| 3 | all old | 100 | 66 | 89 | ||
| Darkness of head and body | 0 | totally white | 0 | 0 | 0 | |
| 1 | reduced grey wash or streaking (confined to flanks and/or single streaks around nape) | 0 | 41 | 0 | ||
| 2 | light streaking/wash to head (incl. some dark around eye); isolated streaks/blotches on body. Overall, body looks more white than brown | 17 | 43 | 22 | ||
| 3 | well streaked: dark mask around eye and/or streaking covering the whole head/face; body with extensive but moderately dense streaks/mottles | 68 | 16 | 78 | ||
| 4 | strong and dense streaking/mottling on body and head making it appear almost wholly dark | 15 | 0 | 0 | ||
| First-generation tertial pattern | 0 | diffuse white tip (like Common Gull Larus canus) | 1 | 65 | 33 | |
| 1 | fine pale fringe around distal portion (like classic michahellis), possibly also with some vermiculations | 3 | 32 | 12 | ||
| 2 | edges moderately notched | 33 | 2 | 22 | ||
| 3 | edges strongly notched and/or some dark barring or pale patches across the feather on some or all tertials | 63 | 1 | 33 | ||
| Second-generation scapular pattern | 0 | uniformly silvery-grey, darker patterning absent or very faint | 0 | 8 | 0 | |
| 1 | silvery-grey background, pattern stronger than on 0, but lacks strong barring or central dark diamonds (only dark shafts and subtle anchors), with only a minority (one or two) of such feathers admixed | 0 | 30 | 0 | ||
| 2 | strong, contrasting shaft-streaks, anchors and/or dark central diamonds, but these more patterned feathers are less than 1/2 of all; ground colour creamy or silvery-grey, possibly with some grey feathers mixed in | 0 | 30 | 0 | ||
| 3 | strong pattern described in 2 on most (more than 1/2) of feathers, but possibly also one or two plain grey feathers or feathers with grey ground tone | 4 | 17 | 12 | ||
| 4 | all feathers contrastingly patterned (with dark cross bars or diamonds), lacking plain grey feathers; feather centres buffy-brown | 96 | 15 | 88 | ||
Larus cachinnans