Larus cachinnans

(last update: February 16, 2013)

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1cy cachinnans: November

Chris Gibbins, Grzegorz Neubauer and Brian Small, in "From the Rarities Committee's files - Identification of Caspian Gull. Part 2: phenotypic variability and the field characteristics of hybrids, (BB 104/2011) present Table 2 in their paper: "Percentages of sample first-winter birds with trait scores for selected plumage, structural and moult characteristics". On this page we repeat table 2 and illustrate each category with example birds. Some traits are very objective (especially measurements from pictures), other traits are very subjective and observers may interpret a single bird in different ways (e.g. leg length).
On this webpage, we repeat table 2 in different order, classified with "most useful traits" on top. The important traits are:
(i) bill length : depth ratio;
(ii) extent of scapular moult;
(iii) greater-covert pattern; and
(iv) primary projection.

(i) bill length:depth ratio, the ratio measured as in adults (for exact measurements, from feathering on top of bill to bill tip).

"0": measured 2,98	"1": measured 2,75	"2": measured 2,16	"3": measured 1,86

Score	Description	Herring (n-85)	Caspian (n=63)	Hybrid (n=12)
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

(ii) extent of scapular moult (note: moult may progress slowly between October - March and therefore be careful when comparing 1W October samples to 1W March samples).

"0" 2CY Caspian Gull February	"1" 1CY Herring Gull December	"2" 2CY Herring Gull January

Score	Description	Herring (n-85)	Caspian (n=63)	Hybrid (n=12)
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

(iii) greater-covert pattern.

"0": 1CY Caspian Gull	"1": 1CY Caspian Gull	"2": 1CY Caspian Gull	"3": 1CY Herring Gull

(iv) primary projection.

"0": 0,63 in Caspian	"1": 0,55 in Herring	"2": 0,44 in Herring	"3": 0,37 in Herring

(v) ventral bulge (discussion on this trait of course are: "what is the division line?" and "is there a difference in birds in relaxed pose or alert individuals?").

"0": Caspian Gull	"0"?: Caspian Gull	"0"?: Caspian Gull	"1": Caspian Gull

(vi) first generation tertial pattern.

Scores for 1st gen tertial patterns in 1W cachinnans.
"0": Caspian Gull	"1": Yellow-legged Gull	"2": argentatus	"3": argenteus

(vii) leg length (discussion on this trait of course are: "what is the division line?" and "is there a difference in birds in relaxed pose or alert individuals?").

"0": Caspian Gull	"1": Caspian Gull	"1": Caspian Gull	"2": Herring Gull

(viii) darkness of head and body.

"1": Caspian Gull	"2": Caspian Gull	"3": Caspian Gull	"4": Herring Gull

(ix) second generation scapular pattern (note: from late November, plain grey upper scapulars normally are 3rd generation feathers, not 2nd generation; and by March many birds in fact show extensive 3rd generation feathers, not 2nd generation).

"0": Caspian Gull	"1": Caspian Gull	"2": Caspian Gull	"3": Caspian Gull	"4": Caspian Gull

(x) median covert moult (from michahellis we know moult is normally clustered in the inner coverts and upper tertials, but may extend outwards. Nonetheless, coverts hidden by flank feathers can, by definition, not be scored, so safely ratio = new feathers / visible feathers).

"1": Caspian Gull	"2": Caspian Gull	"3": Caspian Gull	"4": Caspian Gull	"5": Caspian hybrid

(xi) greater covert moult (as median coverts; ratio = new feathers / visible feathers).

"0": Yellow-legged Gull	"2": Caspian Gull	"3": Caspian Gull	"4": Caspian Gull	"5": Caspian hybrid

(xii) tertial moult.

"0": Caspian Gull	"1": Caspian Gull	"2": Caspian Gull	"3": Caspian Gull hybrid