First-winter Caspian Gull, Czech Republic, March 2011
Part 2: phenotypic variability and the field characteristics of hybrids
Chris Gibbins, Grzegorz Neubauer and Brian J. Small
Abstract Variability in large gulls creates problems for field observers, not least because of the possibility that hybridisation might be responsible for the appearance of what are perceived to be atypical individuals. However, without detailed information on the phenotypic variability in birds of known provenance, it is difficult to resolve debates about the identification of such atypical birds. This paper is the first attempt to develop a quantitative system to address this problem. We use numerical scores from a sample of 404 birds (including pure and hybrid individuals) to describe objectively patterns of variability in the structural and plumage traits traditionally used to identify Caspian Gulls Larus cachinnans. Our results lend statistical support to many of the traits used to identify Caspian Gulls but also indicate that some traits are less useful than previously thought. There was considerable overlap between Herring L. argentatus and Caspian Gulls in the scores for individual traits, but little or no overlap in summed values. While many hybrids had intermediate sum scores, others overlapped with one of their parents in certain characters, indicating that they cannot be identified with any degree of confidence. This emphasises the need for caution when dealing with Caspian Gull records. There are limitations to our approach and there remains a need for more data on hybrids. Nonetheless, the paper provides an objective framework for the treatment of less typical Caspian Gulls and suspected hybrids. We suggest specific threshold scores that can be used as an aid to records assessment, to separate acceptable birds from those which show a combination of traits that make their identification unsafe. Application of the scoring system requires careful observation and good-quality photographs, so it may not always be possible to score candidate birds. We discuss how incompletely documented birds can be treated by observers and records committees.
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