Larus cachinnans

(last update: February 16, 2013)

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sub-adult cachinnans: February

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

BELOW: PART 2 OF THE PAPER

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

Approach

Sample birds and scoring system
The paper is based on studies of Caspian Gulls in the Ukraine, Romania, Latvia, Lithuania and Poland, and Herring Gulls in Britain and Poland (fig. 1). Owing to the availability of data and the extreme variability of other age groups, we focus on adult birds and those in first-winter (1W) plumage (defined further below). However, some information on the variability of birds in juvenile plumage is also presented.

We have been able to examine large numbers of breeding adults in the hand, through work in the Ukraine and on the mixed colonies in Poland; this has allowed us to develop criteria based on detailed in-hand examination of primary and bare-part patterns. Adult gulls trapped in mixed colonies have been assigned to species based on a combination of genetic markers (see Gay et al. 2007) and phenotypic traits. In the best-studied colony, in Poland, individuals intermediate between Herring and Caspian Gull accounted for 27-35% of all birds; mixed pairs involving pure individuals of the two species occurred fairly regularly (c. 15% of all pairs), while pairs involving at least one intermediate individual were even more frequent (40-50%; Neubauer et al. 2009).

Fig. 1. Location of sampling areas relative to the distribution of Herring Larus argentatus and Caspian Gulls L cachinnans. For completeness, the breeding range of Yellow-legged Gull L michahellis is also shown. The distributions are taken from Malling Olsen & Larsson (2003). However; for Caspian Gull we have added shading (light green) to include recently colonized areas in Poland. This species has also colonized inland Russia, Belarus and Lithuania, and is known to breed in eastern Germany, but the paucity of data from these areas means that the distribution shown in this figure is best treated as approximate. The two black lines delimit the approximate hybrid zone, based on the best available current knowledge.

The choice of study locations and data collected in each location was based on our desire to sample hybrids from mixed-species colonies (e.g. Poland) and pure individuals from core areas, away from the hybrid zones (e.g. Caspian Gulls in Ukraine and Romania). However, practical constraints dictated that sampling from core areas was not possible for all age classes. For example, GN lives in Poland, where he has the necessary licenses to handle birds and take blood samples. Thus, out of necessity, sample data on adult Herring Gulls are from his long-term, mixed-species study colonies in Poland. To increase our sample size, data from 1W birds photographed in core areas were supplemented with data from Poland and Latvia. Ringed Caspian Gulls in the sample came predominantly from the core range in the Ukraine, where the influence of other species should be minimal. Ringed birds of known provenance in the sample showed patterns similar to unringed ones, so we have no concerns that the inclusion of Herring and Caspian Gulls from the hybrid zone undermines our general conclusions.

Birds ringed in the hybrid zone were considered hybrids based on certain knowledge of both parents (through observations of nest attendance). Parentage was not confirmed genetically, other than for one individual (depicted in plate 407). However, preliminary results of an ongoing study in the colony where these hybrids come from indicate that cases of extra-pair young (off-spring that are fathered outside the pair bond) are not common (c. 15% among over 200 chicks from 62 families; M. Zagalska-Neubauer and G. Neubauer, unpubl.). Thus, we assume that the likelihood of an extra, unknown adult attending the nest is low.

6CY hybrid Caspian x Herring Gull, Pohlsche Heide, Germany, January 21 2010. Picture: Armin Deutsch. Mother known as Herring and father as Caspian; ringed as a chick in central Poland on May 14 2005. This hybrid is strikingly Caspian-like, particularly the elegant structure and slim bill, pure white head, and P10 with less black than white and a virtually pure white tip. However, the P10 tongue was grey and the eyes pale and unspotted. Its overall score was 9, and most UK birders would probably identify this bird as a Caspian Gull.

7CY male hybrid argentatus (mother) x cachinnans (father), 7CY, ringed as chick in 2003. Wing-tip very close to cachinnans. April 29 2009, Wloclawek - Vistula river, Poland. Picture: Magdalena Zagalska-Neubauer.

In order to assess phenotypic variability quantitatively, and hence the frequency of extremes, we developed a scoring system that allows patterns of variability in adult and 1W birds to be described in a multivariate way. Similar approaches have been used previously to produce so-called ‘hybrid indices’, where there is a need for formal and objective assessment of the position of a particular individual between parental (pure) species in ‘phenotypic space’ (see Bell 1997 and Good et al. 2000 for examples of a similar system of scoring for Western L. occidentalis and Glaucous-winged Gulls L. glaucescens in the Pacific hybrid zone). Application of the system involves scoring individual birds based on a number of traits. The traits we chose to include (table 1 & table 2) are only those that are generally regarded as being useful in the identification of Caspian Gull. This allowed us to maximise between-species differences, since using traits known not to differ (or to differ only slightly) would obscure patterns. It is therefore important to recognise that the traits do not represent a full phenotypic characterisation of the species and that the approach focuses on only the most divergent traits.

For each trait, we developed scores that reflect observed patterns of variability. The system was intended to allow the separation of Herring and Caspian Gulls and their hybrids. Scores were intentionally polarised, with high scores corresponding to Herring Gull and low scores to Caspian. The system is categorical and so cannot fully capture the true nature of the continuous variability observed for each trait. It would be possible to develop a more complex system (e.g. with more categories within each trait in order to better capture subtle variability) but, because we wanted to produce something that was relatively simple and useful for birders in the field, we kept the number of categories to a minimum. For the same reason, we avoided the use of absolute measurements and detailed biometrics; traits that involve assessment of length or other dimensions are all expressed in relative terms (i.e. as ratios).

END OF PART 2

CONTINUE PART 3: ADULT BIRDS