Larus cachinnans

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In 2011, Chris Gibbins, Grzegorz Neubauer and Brian Small published two extensive papers in Britsih Birds, dealing with Caspian Gull. Below, you will find the content of the second paper "phenotypic variability and the field characteristics of hybrids".

The full title reads: 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. ORDER PAPER COPY!

"we" in the text below refers to the original authors. If any errors occur in this text, please let me know and mail to marsmuusseatgmaildotcom.

PART 1: ABSTRACT & INTRODUCTION

PART 2: APPROACH

PART 3: ADULT BIRDS AND FIRST WINTER BIRDS

PART 4: DATA ANALYSIS

PART 5: VARIABILITY IN PLUMAGE (OF ADULTS)

BELOW: PART 6 OF THE PAPER

Identification of Caspian Gull. Part 2: phenotypic variability and the field characteristics of hybrids

Fig. 3. Measurements used to derive bill-length to depth and primary-projection ratios for the sample of first-winter birds. Bill ratio is calculated as L/D, primary projection as I/L (projection beyond tail/length beyond tertials). Both images show same juvenile Caspian Gull Larus cachinnans, August 23 2008, IJmuiden, the Netherlands. Picture: Mars Muusse.

Identification of first-winter birds

Table 4 presents the frequency of (different trait scores for birds in first-winter plumage (in October to March inclusive; see above). These frequency statistics form the basis of the following text.

Variability in structure
Primary projection proved to be a very useful means of distinguishing between the species. Caspian Gulls typically had a long projection of the primaries beyond the tail, with the majority of birds (72%) having a ratio >0.6 (trait score 0). The majority of Herring Gulls had either a medium (score 2) or a moderately long (score 1) projection (45% and 38% of birds respectively). However, some extremely long-winged Herring Gulls occurred in the sample, with 6% of birds having a projection of >0.6, while some Caspian Gulls (3%) had only a medium primary projection, so there is a degree of overlap. Nonetheless, it is clear that a bird with a short primary projection (ratio <0.4) is extremely unlikely to be Caspian.

Contrary to much of the published literature, the presence of a ventral bulge did not prove (quantitatively) useful for distinguishing between the species. It was not uncommon for Caspian Gulls to lack a ventral bulge (38%), while some Herring Gulls (at least in some stances) appeared to show one (14%). The statistics indicate that it would be misleading to refer to a Herring Gull with a ventral bulge or to a Caspian Gull without one as extreme or unusual.

2CY Caspian Gull, February 19 2011, Ashdod, Israel. Picture: Amir Ben Dov. Its plumage is very typical of Caspian Gull - for example, its greater coverts have a trait score of 1, a pattern seen in 63% of birds.

2CY Caspian Gull, Riga, Latvia, March 2009. Picture: Chris Gibbins. This bird has a rather robust bill (measured 2,1), which overlaps with Herring Gull. Such large (presumed) male birds can appear very unlike the caricatured image sometimes painted of Caspian Gull. Its plumage is very typical of Caspian Gull - for example, its greater coverts have a trait score of 1, a pattern seen in 63% of birds. This bird has an overall score of 19, very close to the average for the species (18.2).

2CY Herring Gull (argenteus), February 28 2008, Boulogne-sur-Mer, France. Picture: J-M Sauvage. This is an example of a bird with moderately long legs (trait score 1) and a fine bill that lacks a marked gonydeal angle (score 1). Its plumage, however, is typical. Its overall score is 31, close to the average for this species (33.0) and therefore some odd structural traits do not affect its sum score markedly.

1CY Herring Gull (argenteus), December 09 2012, Hoek van Holland, the Netherlands. Picture: Herman Bouman. Classic bird, regarding head pattern, notching on coverts and tertials, pattern of 2nd gen scapulars and bill ratio. Its overall score is 34 (or 35), higher than the average for this species (33.0), expressing the typical features for Herring Gull in this bird.

Variability in plumage
While the Herring Gulls in the sample had a rather consistent pattern on the greater coverts (all birds were either trait score 2 or trait score 3), Caspian Gulls varied considerably. The most frequent plumage (63%) was the textbook pattern of trait score 1: a pale bar formed by the contrast between the generally dark basal and pale distal portions of each feather (plate here & here). Another frequent pattern in the greater coverts of Caspian Gulls (30% of sample birds) was a delicate “piano key” pattern or vermiculation along the whole feather edge, with the remainder of the feather being dark (trait score 2). However, 4% of the sample birds had a pattern which was much more typical of Herring and Yellow-legged Gulls L. michahellis (trait score 3), with bold notching along the feather (see plate 417). At the other extreme, some Caspian Gulls (3%) had simple pale fringes around each feather, lacking any notching (trait score 0). None of the sample Herring Gulls showed this pattern, although we have seen it on Herring Gulls in the field on rare occasions.

Scores for greater covert (GC) patterns in 1W cachinnans.
cachinnans score 0	cachinnans score 1	cachinnans score 2	argenteus score 3

The tertial patterns on the sample Herring and Caspian Gulls differed markedly. Trait score 3 was typical of Herring (63%), and score 0 typical of Caspian (65%); thus, typically, the species were very different. Nonetheless, variability meant that a significant proportion of the sample Caspian Gulls (32%) had a pattern more like that of Yellow-legged Gull (trait score 1) and some had tertials which were moderately (2%) or strongly (1%) notched (scores 2 and 3 respectively), patterns typical of Herring Gull. Conversely, some Herring Gulls (c. 1%) had the diffuse pale tip that is typical of Caspian.

Scores for 1st gen tertial patterns in 1W cachinnans.
cachinnans score 0	cachinnans score 1	argentatus score 2	argenteus score 3

The pattern on the second-generation scapulars differed overall between the species. A significant proportion of the Caspian Gulls had trait score 0, 1 or 2 for their scapulars (68% of the sample), patterns characterized by simply marked feathers lacking strong diamonds or anchors, with numerous wholly grey feathers present. The textbook Caspian Gull pattern (e.g. plates 418 & 419) was different from that seen in the majority of Herring Gulls, in which trait score 4 was predominant (95% of birds). However, an important point is that a significant proportion of 1W Caspian Gulls had strongly patterned second-generation scapulars (32% of the sample had a pattern scored as 3 or 4).

Contrary to common perception, it is therefore not at all unusual for Caspian Gulls to have second-generation scapulars patterned with strong basal diamonds and/or subterminal anchors. In contrast to that of Herring Gull, the pattern frequently differs across these second-generation feathers, with a mixture of well-marked and plainer scapulars being a common feature of Caspian. The uniform silvery or grey scapular pattern seen in many Caspian Gulls (trait score 0 and 1) was not recorded in any of the sample Herring Gulls.

1CY Caspian Gull November 15 2012, Rødvig-Stevns, Sjælland, Denmark. Picture: Lars Krogh. Pattern on the second-generation scapulars differed overall between the species. A significant proportion of the Caspian Gulls had trait score 0, 1 or 2 for their scapulars (68% of the sample), patterns characterized by simply marked feathers lacking strong diamonds or anchors, with numerous wholly grey feathers present.	1CY Caspian Gull, September & October 2010, Rødvig-Stevns, Sjælland, Denmark. Picture: Lars Krogh. Ringed as pullus in Ukraine in 2010, the core for cachinnans; nontheless, pattern on coverts and head shape reminiscent of michahellis.
1CY Caspian Gull, Sijazan, Azerbaijan, September 2011. Picture: Chris Gibbins. The greater coverts are heavily chequered, quite unlike those of a textbook Caspian. Birds such as this one, in the species’ core range, show that individuals far from the hybrid zone can exhibit such traits. This bird and all September birds were excluded from the sample because they were photographed before the completion of post-juvenile moult.	2CY Caspian Gull, March 08 2011, Ashdod, Israel. Picture: Amir Ben Dove. A bird with textbook colour and pattern in its second-generation scapulars. The number of wing-coverts included in the post-juvenile moult varied from 0 to more than 75% in our sample of Caspian Gulls; this bird has all coverts juvenile.
1CY Caspian Gull,December 15 2011, Blaringhem, France. Picture: Harry Vercruijsse. The number of wing-coverts included in the post-juvenile moult varied from 0 to more than 75% in our sample of Caspian Gulls; this bird shows a typical pattern, with some new inner greater coverts and a scattering of new median and lesser coverts.	2CY Caspian Gull, February 19 2012, Zagreb, Croatia. Picture: Mars Muusse. This bird’s second-generation scapulars score 0 (uniformly silvery-grey, patterning absent or faint). Although regarded as the ‘textbook’ scapular pattern, it is actually not common - only 8% of Caspian Gulls show this pattern.

Our sample showed that it is not uncommon for Caspian Gulls to appear rather heavily streaked on the head and body in 1W plumage. While most birds were much whiter overall than Herring Gulls (84% of Caspian Gulls scored 1 or 2), 16% were as well streaked as a typical Herring Gull (i.e. trait score 3). Some (e.g. PUSZ) were extremely heavily streaked and so quite unlike the search image that most birders have of Caspian Gull.

Fig. 8. Sum trait scores for first-winter Caspian Larus cachinnans and Herring Gulls L. argentatus, and hybrids. Scores are calculated by summing individual trait score values. The figure shows the percentage of individuals in the sample with a given sum value. Numbers in parentheses are the minimum and maximum values recorded.

Fig. 9. PCA biplot showing sample first-winter Herring Larus argentatus and Caspian Gulls L. cachinnans and a number of proven and suspected hybrids. Each circle represents a sample bird. Together, axes l and 2 capture 73.4% of the variance in the sample data. The polygon to the right groups sample Caspian Gulls, while that to the left groups Herring Gulls. Hybrid birds are highlighted in red; numbers relate to plates that show examples of some of these hybrids. Codes for traits are: Bill=bill length:depth ratio, Pproj=length of primary projection, TM=extent of tertial moult, GCovM=extent of greater-covert moult, MCovM=extent of  median-covert moult, ScapM=extent of scapular moult, GCovPat=greater-covert pattern, Tpat=tertial pattern, Legs=leg length,Vbul=presence of ventral bulge, Darkness=extent of streaking on head and body, and ScapP is the pattern on second-generation scapulars.

END OF PART 6

CONTINUE PART 7: Variability in extent of the post-juvenile moult