Larus cachinnans

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sub-adult cachinnans: October

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)
PART 6: IDENTIFICATION OF FIRST WINTER BIRDS
PART 7: VARIABILITY IN EXTENT OF POST-JUVENILE MOULT
PART 8: UNDERWING PATTERN
PART 9: DISCUSSION

BELOW: PART 10 OF THE PAPER - KNOWLEDGE GAPS

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

Knowledge gaps
Our understanding of Caspian Gull has improved dramatically thanks to the pioneering work undertaken in the 1990s by Ronald Klein, Lars Jonsson, Martin Garner and others. Research published since then has provided further insights into the ecology, genetics and identification of Caspian and related gulls. Despite the advances, these gulls still present challenging biological questions (for example, related to their evolution; Liebers et al. 2004) and for birders they will no doubt continue to pose challenging identification problems. While this paper will not allow birders to identify all the problem Herring/Caspian Gulls they encounter, we hope that it at least provides a structured framework that can be used to reach objective decisions about which ones are identifiable and which are not. It is certainly the case that many of the hybrids in our sample look intuitively odd, appearing neither like pure Herring nor like Caspian. Birders who spend time regularly looking at large gulls are most likely to sense this overall intermediacy; the scoring system allows this to be quantified and reported objectively.

In terms of describing patterns of variability, this paper should be seen as a first step. It is largely the result of informal observation and analysis, rather than funded, professional scientific research. Constraints on the locations we have been able to visit, relative to the full range of the species, mean that we cannot present a complete picture of intra- and interspecific variability, nor the variability of hybrids. Specific issues related to this, and other aspects of large gull identification that require further work, are discussed below.

1CY backcross hybrid Caspian x Herring Gull 46P1 September 26 2006, Simrishamn, Sweden. Picture: Jörgen Bernsmo. Ringed as pullus on May 17 2006 at Wloclawek Vistula river, Polanda, mother known as Caspian, father as a hybrid Caspian x Herring Gull. This is a very difficult bird, combining Caspian jizz (long legs, slender bill) with aspects of plumage and moult that are more like Herring. Its score (25) is too high for Caspian and low for Herring and should flag it as a possible hybrid.

3CY backcross hybrid Caspian x Herring Gull 4P46 November 03 2006, Detmold, Germany. Picture: Armin Deutsch. Currently, we have too little information to develop identification criteria for hybrids of 3W age class. This bird has more uniform grey on its upperparts than on a typical Herring of this age, a longish bill but moderate (too heavy for Caspian) streaking on the head. The iris is much paler than typical in Caspian of this age.

5CY backcross hybrid Caspian x Herring Gull 4P46May 21 2008, Włocławek Reservoir, central Poland. Picture: Grzegorz Neubauer. Given its age, this bird shows rather a surprisingly brownish alula and primary coverts. With a score of 10, it falls closer to Caspian than Herring, as expected given that Caspian genes predominate. Its hybrid origin is unlikely to be detected in the field, and most likely this bird would be identified as Caspian if unringed.

7CY backcross hybrid Caspian x Herring Gull 4P46 October 13 2011, Detmold, Germany. Picture: Armin Deutsch. This interesting plate shows 4P46 as an adult (7CY), nearly in full winter plumage (primaries still regrowing).The head-streaking, especially the shadowing around the eye, is too heavy for any normal Caspian Gull, and perhaps would be the first clue to it being a probable hybrid. Note than in this photograph the bill is slightly open and so looks deceptively deep.

1. Hybrids and sample size
Although we included as many hybrids in our sample as possible, the numbers are small and that sample is unlikely to capture the full variability in the appearance of hybrids. More work is needed to record the appearance of hybrids and assess the extent to which our data are representative. Heterospecific pairs, where the partners represent pure individuals of different species, constitute only a minority in mixed colonies because of the isolation mechanisms which limit mixed-species pairing (Neubauer et al. 2009) - and because of that it is clear that a full understanding of the phenotypic variation of hybrid individuals is some way away.

2. Yellow-legged Gull
The scoring system we have developed does not deal with Yellow-legged Gulls. Generally, this species is not a major cause of confusion for observers faced with a putative Caspian Gull in Britain, but some birds create problems. Work is needed to develop a scoring system to separate Yellow-legged from Caspian Gulls, and hybrids between these two.

Larus michahellis 1CY PFAD September 18 2008, Ares beach - A Coruña, NW Spain. Picture: Antonio Gutierrez. Ringing data: PFAD green ringed as chick at Delta del Ebro, Tarragona on 18 June 2008.	Larus michahellis 2CY ITO CF4024 October 25 2008, Etaples, NW France. Picture: Theo Muusse. Ringed as pullus on May 21 2007 at Isola di Bergeggi, Savona, Italy (44,24 08,44).
Larus michahellis adult, January 02 2011, Mamaia, Romania. Picture: Chris Gibbins.	Larus michahellis adult, 19 February 2012, Zagreb, Croatia. Picture: Mars Muusse.

3. Long-term studies and the identification of other age groups
We have dealt only with adult and first-winter birds and there is a need to collect data on other age groups. While most birds will not be problematic, others are genuinely difficult and a quantitative approach is most likely to provide the insights needed to resolve the identification of the more difficult pure individuals of various immature age classes, and hybrids.

If the plumage and plumage development of these long-lived birds is to be studied in nature, research is required in the hybrid zones. So far it seems that intensive colour-ringing of the offspring of mixed pairs yields the most insightful results. Several ringed immature birds have been observed in Europe, and are sufficiently well documented by photographs that details of their phenotype can be studied (e.g. plate 434). The central problem with this approach is that the average probability of recovering a ringed bird is low. Moreover, only a minority of reports of colour-ringed individuals is accompanied by photographs, and very few of these have the quality necessary to provide insights into subtle plumage or structural details. Another difficulty is that of confirming the identification of both parents of ringed chicks. This requires lots of time spent observing adults at nests and trapping more problematic individuals. Hence, while following the plumage development of hybrids ringed as chicks will likely yield the required information, this work will inevitably be slow.

END OF PART 10

CONTINUE PART 11: KNOWLEDGE GAPS 4.