Acrocephalus and Hippolais Warblers from the Western Palearctic David T

Species limits in Acrocephalus and Hippolais warblers from the Western Palearctic David T. Parkin, Martin Collinson, Andreas J. Helbig, Alan G. Knox, George Sangster and Lars Svensson Sedge Warbler Acrocephalus schoenobaenus Richard Johnson ABSTRACT The taxonomic affiliations within the genera Acrocephalus and Hippolais have long been a matter for debate. Recent molecular and behavioural studies have provided a wealth of new data which can be used to analyse the evolutionary relationships of the Palearctic taxa in these genera. In this paper, we make a series of recommendations for changes in species limits, highlight some problem areas and discuss situations where more research is needed. 276 © British Birds 97 • June 2004 • 276-299 Species limits in Acrocephalus and Hippolais warblers Introduction dering array of small and difficult birds (for long period of taxonomic stability fol- example Emei Leaf Warbler Phylloscopus lowed the publication of the ‘Voous List’ emeiensis, Alström & Olsson 1995; the chiffchaff Afor Holarctic birds (Voous 1977), but in complex, Helbig et al. 1996; and the Greenish recent years there have been dramatic advances Warbler P. trochiloides, Irwin et al. 2001a,b,c), in our knowledge of the evolutionary relation- which has led to an improved understanding of ships of birds, leading to a period of activity their evolution, and hence their taxonomic rela- that shows no sign of abating. These advances tionships. This paper draws together some stem largely from new and exciting ways to recent studies which together provide a clearer study birds, both in the field and in the labora- picture of an especially complex group, the tory. Observations from a variety of disciplines, Acrocephalus and Hippolais warblers, and we both professional and amateur, are providing present the reasons for changes to the British valuable data which can be used to resolve List recommended by the TSC (Knox et al. problems in phylogeny and taxonomy. Develop- 2002). The paper is largely restricted to the ments in ecological and behavioural analysis Western Palearctic, only straying beyond its and molecular and population genetics have boundaries where it is necessary to establish the allowed new insights into evolution and phylo- limits of species which occur within it. We have genetics, and these have combined with a followed the criteria discussed by Helbig et al. revised view of the species concept to generate a (2002), attempting to determine whether there significant reappraisal of the taxonomic rela- is evidence that the individual taxa form sepa- tionships of many of the species that we rate evolutionary lineages between which there thought we knew so well. is little or no gene flow. Where we find clear dif- The BOU Records Committee’s Taxonomic ferences between the taxa, we treat these as sep- Sub-committee (TSC) has recently reported on arate species; if they show some divergence, but its approach to the recognition of species are not diagnosably distinct in at least two inde- (Helbig et al. 2002; Parkin et al. in press). Essen- pendent characters (e.g. morphology, vocalisa- tially, the TSC regards a species as a population tions, DNA), we treat them as subspecies. lineage which maintains its integrity through There are many publications relating to the space and time: the ‘General Lineage Concept’, identification of Acrocephalus and Hippolais advocated by de Queiroz (1999). As such, a warblers, and the field characters of many species is defined as a group of populations species are now well established. Not infre- which is identifiably (diagnosably) different quently, however, a ‘mystery’ bird is discovered from other such groups; is reproductively iso- which does not fall easily into a predetermined lated from them; shows substantial divergence category. Observers attempt to determine its from them at a genetic level; and whose identity, often by elimination (e.g. Dunn 2001), members share a common mate-recognition and with the help of museum skins, pho- and fertilisation system (Helbig et al. 2002; tographs and experts. Typically, the conclusions Parkin 2003). Diagnosability may be morpho- arrived at cannot be tested, because the original logical (plumage or structure) or molecular bird is no longer available. In most cases, the (absolute differences in DNA sequence). correct identification is probably established Genetic divergence implies that, although the but such an approach is dangerously close to a genetic structure is not diagnosably distinct, circular argument and, although useful, is cer- there are differences in gene frequency which tainly not scientific. Only by ground-truthing indicate a prolonged period without gene flow. the conclusions with individuals of known Species-recognition systems require behav- identity can the criteria developed in this way ioural or acoustic characters which prevent, or be tested. This is rarely done. More frequently, at least substantially reduce, the likelihood of the characters used to determine the specific hybridisation. In a particular species group, identity of an individual vagrant become these data may come from a wide range of enshrined into knowledge, or even folklore, sources such as behavioural, ecological or mole- until further evidence casts doubt on these cri- cular analyses, field observations or biometric teria. Such are the advances being made within data collected by ringers. difficult groups, including Acrocephalus and Recent research on the warblers (Sylviidae) Hippolais warblers, that many established cri- has provided a wealth of new data for a bewil- teria are in a state of flux, with new develop- British Birds 97 • June 2004 • 276-299 277 Species limits in Acrocephalus and Hippolais warblers Ray Tipper 161. Melodious Warbler Hippolais polyglotta, Algarve, Portugal, June 1998. Occuring in the most straightforward of the six ‘groups’ considered in this paper, Melodious and Icterine Warbler H. icterina form an obvious species pair, which is supported strongly by molecular evidence. ments frequently dispelling previously held supported in this way. Where specimens are not misconceptions. preserved, future researchers can never be com- Of greater value is research involving indi- pletely certain to which taxon a particular individuals of proven identity and natal origin. vidual in any study belonged. The fact that a Studies based on birds of known breeding particular observer identified it may not be provenance should be used to generate baseline deemed acceptable in 50 or 100 years time, morphological, vocal or genetic data, providing when his or her skills have been forgotten. If the opportunity for sound statistical analysis specimens are not taken for permanent storage, and interpretation. Research based on birds it is even more important that the exact date trapped or collected on migration is of lesser and location is recorded for every individual value because the population of origin will gen- record. A Willow Warbler P. trochilus trapped in erally be unknown. This review attempts to May at its breeding site in Oxfordshire can at restrict itself to studies conducted in the former least be assigned to a population that may be way. resampled again in the future. The provenance Some of the results discussed here relate to of a Willow Warbler trapped on migration, for field identification, vocalisations or genetics, example in September on Fair Isle, Shetland, or and come from trapping and ringing studies; Helgoland, Germany, is less easily determined. others are based on museum specimens. So, in this review, greater credence is given to Despite the current trend away from collecting studies which utilise museum skins (since these specimens for scientific purposes, there is no can be re-examined in the future) or birds doubt that research based on museum skins has trapped while breeding (because their popula- a major advantage over many other approaches, tion of origin is known). since it is possible for subsequent researchers to The most important reviews of plumage revisit and reassess precisely the same individ- (colour and pattern) and morphology (size and uals. A strong case can be made for professional shape) include those by Baker (1997), Beaman scientific research to require the collection of & Madge (1998), Cramp (1992), Glutz von birds for studies of morphology or genetics, and Blotzheim & Bauer (1991), Harris et al. (1996), their permanent and secure storage (as voucher Lewington et al. (1991), Shirihai et al. (1995, specimens) for the benefit of future researchers. 1996), Svensson (1992, 2001), Williamson Indeed, some journals maintain this practice, (1960) and Zimmerman et al. (1996). Several of and will only accept scientific reports which are these are identification guides, however, and 278 British Birds 97 • June 2004 • 276-299 Species limits in Acrocephalus and Hippolais warblers tend to emphasise differences rather than conti- nuities in phenotype. Behavioural and vocalisa- tion studies include those of Lemaire (1977), Dowsett-Lemaire & Dowsett (1987) and Svensson (2001). Important studies which have utilised DNA sequences include Leisler et al. (1997), Helbig & Seibold (1999) and Bensch & Pearson (2002). Previous genetic research Reference to technical papers by Leisler et al. (1997) and Helbig & Seibold (1999), in which the molecular structure of the Acrocephalus and Hippolais warblers was investigated, will recur throughout this paper, so it is worth devoting a few lines to their scope, strengths and weak- nesses. Leisler

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