Identification of Vagrant Iberian Chiffchaffs – Pointers, Pitfalls and Problem Birds J

BB April 2008 20/3/08 13:27 Page 174 Identification of vagrant Iberian Chiffchaffs – pointers, pitfalls and problem birds J. Martin Collinson and Tim Melling ABSTRACT Records of Iberian Chiffchaff Phylloscopus ibericus in northern Europe are increasing. At the time of writing, all British records have been singing males. In this paper, we present sonograms of some accepted and potential Iberian Chiffchaffs from Britain.The characteristics of Iberian Chiffchaff song that can be used for identification of vagrants are reviewed. A record of a bird at Skelmersdale, Lancashire, in 2004, is thought unlikely to have been an Iberian Chiffchaff, but another bird, a mixed singer at Dibbinsdale, Merseyside, also in 2004, may be acceptable.The vocalisations of an accepted Iberian Chiffchaff in Oxfordshire in 2000 are now considered not to be absolutely typical. A problematic bird at Lavenham, Suffolk, in 2007 is also discussed and thought not to be acceptable. Introduction mantle than Common Chiffchaffs (Salomon The Iberian Chiffchaff Phylloscopus ibericus 1997; Salomon et al. 2003; Slaterus 2007). Other (formerly P. brehmii), a relatively recent split supportive features (statistical tendencies rather from Common Chiffchaff P. collybita (Helbig et than diagnostic traits) are reported to include: a al. 1996; Clement & Helbig 1998; BOU 1999), longer tail, a lemon-yellow wash to the super- has a breeding range limited almost entirely to cilium in front of the eye, a yellow bill-base and the Iberian Peninsula (fig. 1). a less obvious eye-ring (owing to the strong supercilium and plain pale ear-coverts) that is Morphology thicker above the eye than below (Richards There are small but diagnostic differences in 1999; Slaterus 2007). The legs of Iberian Chiff- plumage and biometrics between the two chaff are generally, though not always, dark species, as well as a tendency for different brown. Iberian Chiffchaff has been reported to habitat preferences and a high degree of genetic have a longer supercilium than Common, but divergence (4.6% at the mitochondrial-DNA there is enough variation in both species to level, indicating a long period of evolutionary make this feature unreliable for identification. separation) (Helbig et al. 1996, 2001; Salomon On current knowledge, geographic and indi- 1997). Iberian Chiffchaffs, especially northern vidual variation within both Common and populations, tend to have longer, more pointed Iberian Chiffchaffs suggests that identification of wings than Common Chiffchaffs (giving them a a silent migrant remains a significant challenge more Willow Warbler P. trochilus-like wing and is likely to be virtually impossible in the structure). Iberian Chiffchaffs also have cleaner field. Northern populations of Iberian Chiff- white underparts with a noticeably yellow chaffs are biometrically distinct from southern throat and vent, and a subtly brighter green populations of Common Chiffchaffs on the basis 174 © British Birds 101 • April 2008 • 174–188 BB April 2008 20/3/08 13:27 Page 175 Identification of vagrant Iberian Chiffchaffs of wing size and shape, and tarsus and bill length (Salomon 1997, 2002). However, southern pop- 42° ulations of Iberian Chiff- chaffs have shorter, less pointed wings than northern birds, while northern Common Chiffchaffs P. c. abietinus have long wings similar to southern Iberian Chiffchaffs (Salomon 2002). So although some vagrant Iberian Chiff- chaffs may be struc- turally distinctive, others, 34° especially from southern populations, will not be. In fact, all the ‘distinctive’ © Fluke Art © Fluke plumage and structural 8° 0° 8° features of Iberian Chiff- Fig. 1. Breeding range of Iberian Chiffchaff Phylloscopus ibericus (green shading), chaff overlap with adapted from the map in BWPi (BirdGuides Ltd).The range of Iberian Chiffchaff Common Chiffchaff and overlaps that of Common Chiffchaff P. collybita and has been intensively studied in require extremely careful a narrow, 20-km zone in the western Pyrénées around the France/Spain border, where the two species hybridise (shaded in orange).The most recent data on description. In any case, breeding distribution of Iberian Chiffchaff in Spain is published in Martí & del they are so subtle that, in Moral (2003).The map presented here may require modification, and the extent the context of a rarity of the regular zone of overlap between the two species is not fully known. report based on field observations, they are likely to be sufficiently Hybridisation dependent on light conditions and the expecta- The ranges of Common and Iberian Chiffchaffs tions and perhaps the imagination of the overlap in a narrow, 20-km zone in the western observer as to be almost totally subjective. For Pyrénées around the France/Spain border (fig. example, there is a popular misconception that 1), and the two species hybridise. This is prob- Iberian Chiffchaff has a longer bill than ably a secondary contact zone following range Common Chiffchaff, and observers of extralim- expansion after a period of isolation from each ital Iberian Chiffchaffs in northern Europe have other (Salomon 1989, 2001). Although most reported them as having long bills (Slaterus birds within this overlap zone are assignable to 2007; this paper). However, biometrics show that one species or the other, around 24% of all the bill length of Iberian Chiffchaff is virtually breeding pairs can be shown by molecular and identical to that of Common Chiffchaff and, if morphological analysis to be mixed pairs, anything, is shorter. The bill is, however, often almost exclusively male ibericus with female described as ‘spikey’, perhaps implying a nar- collybita (Helbig et al. 2001; Bensch et al. 2002). rower bill-base. Some birds appear to be intermediates, i.e. pos- sessing one species’ genes but emitting the Voice vocalisations of the other species, and 8.6% of Fortunately, Iberian Chiffchaff has diagnostic birds are mixed singers, giving song bursts that songs and calls, which are discussed in detail consist of elements of songs from both species below. All extralimital birds identified in Britain (Salomon & Hemim 1992; Salomon 1997; and The Netherlands have been singing males Bensch et al. 2002). All these are usually taken in spring (Slaterus 2007). Other age and sex as signs of hybrid origin. Although successful classes are clearly being overlooked, although hybridisation, leading to viable recruits to the females and autumn migrants could perhaps be population, is difficult to prove (Salomon 1987; expected to be identifiable by their calls. Helbig et al. 2001), it has been inferred – 10% British Birds 101 • April 2008 • 174–188 175 BB April 2008 20/3/08 13:27 Page 176 Identification of vagrant Iberian Chiffchaffs a s bc s s ** ** *+†††† + d ***** +††† ++ e s f s g s hi ss Fig. 2. Songs and calls of Common Phylloscopus collybita and Iberian Chiffchaffs P. ibericus. (a, b) Advertising songs of Common Chiffchaff. (a) Fragment of the advertising song of Common Chiffchaff. Sonogram created from song recorded in Schulze & Dingler (2007).This song may be transcribed as ‘prp prp chaff chaff chiff chaff chaff chaff chiff chaff’. Frequency range 3–8 kHz.The shoulder of the ‘chiff’ notes is at 5–5.5 kHz, and of the ‘chaffs’ at around 4 kHz, although there is individual variation. (b) Advertising song of Common Chiffchaff, Britain, June 1976. Sonogram created from song 176 British Birds 101 • April 2008 • 174–188 BB April 2008 20/3/08 13:27 Page 177 Identification of vagrant Iberian Chiffchaffs of birds within the contact zone show genetic files. Initially, sonograms were produced using evidence of a hybrid origin (Bensch et al. 2002). Avisoft-SASLab Light (www.avisoft.com/ Nevertheless, hybridisation is not ‘the norm’ – downloa_.htm), latterly superseded by Syrinx and it can be inferred that (mostly) assortative (developed by John Burt and available from mating and reduced fitness of female hybrids is http://syrinxpc.com/). preventing the two species from merging The advertising song of Common Chiffchaff (Helbig et al. 2001). The two taxa have been is familiar to most European birders, but that of studied intensively as an example of speciation Iberian Chiffchaff is less well known outside its in action (Salomon 2001). restricted geographic range. Mixed singers have also been recorded from the breeding range of Iberian Chiffchaffs outside Common Chiffchaff the hybrid zone, in Portugal and Gibraltar The elements of the song of nominate (Thielcke & Linsenmair 1963; Salomon 1997; Common Chiffchaff are comparatively Bensch et al. 2002), where a direct hybrid origin, invariant (Thielcke & Linsenmair 1963). There though not impossible, is less likely. For the iden- are three or four standard notes, representing tification of a singing putative Iberian Chiffchaff different frequencies of ‘chiffs’ and ‘chaffs’. On in northern Europe, therefore, the issues to be the other hand, there is considerable variation addressed are: in the order, sequence and length of song • How to distinguish a bird with true Iberian elements among Common Chiffchaffs. The Chiffchaff parentage from a Common Chiff- song is long (typically more than 4 s), metro- chaff with an aberrant song. nomic and relatively simple, with frequency • How to demonstrate whether a bird is a range 3–8 kHz. Typically, the ‘shoulder’ of the mixed singer. ‘chiff’ notes is at 5–5.5 kHz, and that of the • Should mixed singers be written off as ‘chaffs’ at around 4 kHz, as shown in fig. 2a. hybrids? Although the song elements in fig. 2a are The purpose of this paper is to publish sono- complex, these song elements may be sung in grams of known Iberian Chiffchaffs, to reiterate simpler form, probably indistinguishable by ear some of the potential variation and diagnostic under most field conditions but, in sonograms, features in the song. Extralimital birds identified lacking a strong terminal flourish (‘dog-legs’) as as Iberian Chiffchaff in Britain are included. We in fig. 2b. The call is an upwardly inflected ‘huit’ also comment on the identification of three rising from about 2.5 to 4.5 kHz (fig.

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