Pale-legged : new to Britain

John Headon, J. Martin Collinson and Martin Cade

Abstract A Pale-legged Leaf Warbler Phylloscopus tenellipes was discovered dead after hitting a window at the lighthouse on St Agnes, Isles of Scilly, on 21st October 2016. Feathers taken from the were sent for DNA analysis, which confirmed the bird to be a Pale-legged Leaf Warbler and eliminated the morphologically similar P. borealoides. An earlier record of one of this pair from Portland, Dorset, on 22nd October 2012 concerned a bird photographed and seen well by several observers, but it was not possible to establish which species was involved. Full details of both sightings are described here. This is the first record of Pale-legged Leaf Warbler for Britain and the Western Palearctic, and the species has been added to Category A of the British List.

Pale-legged Leaf Warbler on Scilly t around midday on 21st October point Pale-legged Leaf P. tenellipes and 2016, Laurence Pitcher (LP) was Sakhalin Leaf Warbler P. borealoides were not A eating a pasty outside the lighthouse species that crossed my mind, but several on St Agnes in the Isles of Scilly when the people responded immediately – notably owner, Fran Hicks (FH), came over for a James Gilroy, Chris Batty and Andrew Holden chat. They bemoaned the lack of migrants on (AH) – with these very suggestions. AH hap- the island, but in parting FH casually men- pened to be on the island and soon arrived at tioned that a Phylloscopus warbler had struck the lighthouse. Over the phone he was able to his conservatory window earlier that check the wing formula and go through some morning and sadly died. FH added that other features with Chris Batty, who was although it resembled one of the Yellow- giving pointers from Williamson’s Phyllo- browed Warblers P. inornatus that had been scopus identification for ringers guide frequenting the lighthouse garden, there were (Williamson 1976). This established beyond aspects of its appearance that were niggling doubt that the bird was either a Pale-legged him. That could well have been the end of Leaf or a Sakhalin Leaf Warbler. There is as the story but, since FH was returning to his yet no reliable means of separating these two garden, LP casually asked if he could see the species in the hand, or in the field unless they dead warbler. FH quickly reappeared, holding are singing or calling. But DNA testing could the corpse. LP takes up the story: conclusively establish a final identification, ‘The bird was not a Yellow-browed and my and whichever species it was, this would thoughts turned to Greenish P. trochiloides or become the first proven record of that species P. borealis. I dismissed in the entire Western Palearctic.’ Greenish but something was not right for This clearly wasn’t a conventional ‘bird Arctic either. I couldn’t reconcile the primary find’; instead it was more of a cooperative projection, and the bird’s bill seemed odd, but effort by the wider birding community and the I admit to overlooking some obvious input of all those involved is greatly appreci- plumage features. I decided to circulate ated. The dead bird was given to Nigel Hudson images on Twitter, hastily captioned “Arctic (then the BBRC Secretary) for safekeeping in Warbler?” and inviting comments. At this his freezer on St Mary’s, which enabled many

438 © British 111 • August 2018 • 438– 445 Pale-legged Leaf Warbler: new to Britain Laurence Laurence Pitcher 298. Pale-legged Leaf Warbler Phylloscopus tenellipes, St Agnes, Isles of Scilly, 21st October 2016. birders on Scilly to pay their respects. The body field only and at present the difficulties of was sent to the British Museum, Natural distinguishing between Pale-legged Leaf and History, at Tring for preparation. Sakhalin Leaf Warblers make it impossible to take this identification further. It is perhaps Discussion ironic that all birders on Scilly at the time There has been one previous example of this would have preferred to see this bird alive, species pair, at Portland, Dorset, in October but if that had happened it would not have 2012 (see below). That bird was seen in the been possible to identify it to species!

Biometrics and wing formula The following biometrics were taken by Jim Askins at Carn Ithen, Old Town, St Mary’s, on 23rd October 2016.

Wing length 61 mm Tail length 45 mm Bill (to skull) 12.5 mm Bill (to feathering) 10 mm Bill width (proximal end nostril) 3 mm Bill depth 2.5 mm (slightly damaged) Tarsus 18 mm Hind claw 5.5 mm Weight 9.8 g Fat score (scale 0–8) 3 Muscle score (scale 0–3) 2

Primary-tip shortfall (mm) measured relative to wing point (WP = longest primary), apart from P1, which is measured relative to primary coverts; e = emarginated primary.

P1 P2 P3 e P4 e P5 e P6 e P7 P8 P9 P10 PC+6 -9 -1 WP -1 -3 -7 -9 -11 -15

Length of each individual tail feather (mm). R6 and R5 showed a white inner tip.

R6 R5 R4 R3 R2 R1 43 45 44 44 44 41

British Birds 111 • August 2018 • 438– 445 439 Headon et al.

Fig. 1. Phylogenetic tree based on the COI gene for the ‘Scilly Phyllosc’ alongside individuals of Phylloscopus tenellipes and P. borealoides from breeding areas, as well as other superficially similar taxa (Arctic Warbler P. borealis, Kamchatka Leaf Warbler P. examinandus and P. xanthodryas). Nodes in red have >99.9% bootstrap statistical support. They delineate a robust split between the borealoides individuals (in blue) and the tenellipes individuals (in yellow), and confirm that the Scilly bird was a Pale-legged Leaf Warbler.

DNA analysis Scilly bird was genetically highly divergent Two small feathers from the bird were sent to from borealoides (and all other species) and JMC for analysis. DNA was extracted using almost identical to previously sequenced the QIAGEN DNA Micro Kit according to individuals of tenellipes. For example, at the manufacturer’s instructions. Genetic sexing COI gene, it was within 2–5 bp (out of 711 using both the P2/P8 and 2550F/2718R PCR bp) of 13 tenellipes, and at least 27 bp (3.8%) primer sets (Griffiths et al. 1998; Fridolfsson different from any of 15 borealoides. Any & Ellegren 1999) yielded single (Z chromo- database birds sampled away from their some) bands, indicating that the bird was breeding grounds were then excluded from a male. The cytochrome b mitochondrial the analysis to avoid circular reasoning (most gene (cytb) was amplified using the had been identified on DNA), and phyloge- L14993/H16065 primers (Helbig et al. 1995) netic trees drawn up using ‘Neighbour and the cytochrome c oxidase subunit 1 gene Joining’ and ‘Maximum Likelihood’ method- (COI) using the primers BirdF1/BirdR1 ologies – they all told the same story and (Hebert et al. 2004). Both genes were placed the ‘Scilly Phyllosc’ in the tenellipes sequenced and were compared with database group with >99.9% statistical support. A rep- sequences of tenellipes and borealoides previ- resentative tree is presented as fig. 1. The COI ously obtained by other groups from the sequence was submitted to Genbank, with breeding and wintering grounds. It became accession number MG917724. immediately obvious for both genes that the

John Headon*, Hivernia, Jackson’s Hill, St Mary’s, Isles of Scilly TR21 0JZ; e-mail [email protected] J. Martin Collinson, University of Aberdeen, School of Medicine, Medical Sciences and Nutrition, Institute of Medical Sciences, Foresterhill, Aberdeen AB25 2ZD; e-mail [email protected]

* John Headon has compiled this article in his capacity as the Isles of Scilly Recorder and would like to emphasise that he was not involved in the identification of this bird.

440 British Birds 111 • August 2018 • 438– 445 Pale-legged Leaf Warbler: new to Britain Pale-legged Leaf Warbler or Sakhalin Leaf Warbler in Dorset

onday 22nd October 2012 was a mind the enormous interest there would be day when perfect fall conditions from the birding community, it was agreed Mdropped a major arrival of birds on that if the bird was present in the morning I the Isle of Portland, Dorset. Pete (PS) and would attempt to trap it and release it in a Debby Saunders were, as usual, attentive to the suitable nearby spot where it might be acces- comings and goings in their garden at South- sible for general viewing. well and, with one or two Yellow-browed War- In the event, further research quickly blers already seen that morning, they were revealed that the identification was not as alert to rarity possibilities. Towards midday, PS straightforward as I had originally thought: had a brief glimpse of a Phylloscopus warbler initial indications that Pale-legged Leaf with a striking supercilium in low vegetation Warbler might be separated from its sister beside the garden pond. Being unable to put a species, Sakhalin Leaf Warbler, by, for name to it, he summoned assistance from example, the presence of strong wing-bars Grahame Walbridge, who was birding nearby; (del Hoyo et al. 2006) or paler legs (Brazil on arrival, Grahame suggested that the bird 2009) were contradicted by other sources. It might be an P. coro- was soon apparent that the two species may, natus. With only two previous British records, in fact, be separable only by subtle morpho- it was quickly apparent that there would be logical features, by song, or by genetic exami- considerable interest in the bird and local nation (Carey et al. 2001). There was clearly a birders were invited into the garden. The bird more pressing need for the bird to be trapped remained rather faithful to a single large and than just to make it available for general still very leafy Sycamore Acer pseudoplatanus, viewing. Sadly, this was not to be possible as where it was relatively easily seen, although it departed overnight. most views were brief and into the light and, With the benefit of hindsight, the initial unless the bird was watched from some dis- confusion over the bird’s identity became tance, invariably from below. A total of 19 more understandable. It also provided a valu- observers came and went during the after- able lesson in the worth of securing tangible noon, with all securing at least some views of evidence in support of a sighting, rather than it. Towards dusk it moved lower and it was at relying on the belief that what was thought to this time that Brett Spencer voiced a feeling have been seen with the eye must accord with that the bird might in fact be an Arctic reality. In subsequent conversations with Warbler. observers of the bird, I was more than once That evening, PS reviewed his pho- greeted with expressions of incredulity that it tographs of the bird and discovered that possessed neither a crown-stripe nor yellow none showed a crown-stripe – surely, there- undertail-coverts – these observers felt sure fore, it could not be an Eastern Crowned that they had seen one or both of these fea- Warbler. I became aware of doubt sur- tures. In truth, a combination of factors had rounding the identification but was unable to resulted in circumstances that were far from review my photographs until later in the perfect for critical observations: evening. On eventually doing so, I discovered G The bird was extremely active and rarely a suite of features that quickly eliminated visible for more than a few seconds at a both Eastern Crowned and Arctic Warbler; time, and at some distance. only after a literature search (Ticehurst 1938; G The light was always poor and most close Williamson 1976; Leader 1993; Brazil 2009) views were from underneath. did I discover what I took to be the bird’s G Having been kindly invited into a private true identity: a Pale-legged Leaf Warbler, and garden, many observers perhaps felt obliged as such not only new for Britain but also the not to overstay their welcome and to make first record for the Western Palearctic. This space for others, and were happy just to see revelation was relayed to PS and, bearing in the bird briefly rather than to hold out for

British Birds 111 • August 2018 • 438– 445 441 Headon et al.

an opportunity to scrutinise it at length. was estimated to be approximately three- Without the photographic evidence, an quarters of the length of the exposed tertials. erroneous Eastern Crowned Warbler identifi- cation may have been perpetuated. Remark- Head pattern The supercilium was striking, ably, in the context of such an important appearing relatively weak between the nos- record, and even with this photographic evi- trils and the eye but broadening above and dence, the bird’s true identity to species level behind the eye and, depending on posture, may never be ascertained. often noticeably flared upwards towards the nape. Although washed darker before the eye, Description it was whitish for the rest of its length and Size and structure A bulkier Phylloscopus was the palest and brightest component of than the accompanying Common Chiffchaffs the bird’s plumage. The broad dark eye-stripe P. collybita but otherwise unexceptional in which bordered the length of the supercilium structure: deeper billed than a Chiffchaff but thickened conspicuously behind the eye. A the bill did not appear unduly long or heavy, thin off-white eye-ring below the eye-stripe and legs were not noticeably strong or long. was often apparent, although never as con- From photographs, the primary projection spicuous as on the accompanying Chiffchaffs. Pete Pete Saunders Pete Pete Saunders Pete Saunders 299–301. Pale-legged Leaf Warbler Phylloscopus tenellipes or Sakhalin Leaf Warbler P. borealoides at Southwell, Portland, Dorset, 22nd October 2012.

442 British Birds 111 • August 2018 • 438– 445 Pale-legged Leaf Warbler: new to Britain

The ear-coverts were greyish and blotched scopus warblers given decent views. Eastern with small paler spots. The crown was uni- Crowned Warbler is also grey-crowned but formly greyish-olive (noticeably darker than always has a paler central crown-stripe, yellow the mantle), with a suggestion of darker undertail-coverts and darker legs. Arctic shading sometimes visible at the sides above Warbler is superficially similar but is larger the supercilium. and longer billed, has a creamy-yellow super- cilium that reaches only to the lores, the crown Mantle and wings Dull olive, often looking concolourous with the mantle, and yellowish brightest – almost bronzy in some light – on legs. Of the likely confusion species, Two- the edges of the flight feathers and tertials. barred P. plumbeitarsus The primary coverts were noticeably darker, most closely resembles Pale-legged Leaf and and at times appeared almost black. Two dull Sakhalin Leaf Warblers in plumage, although creamy-yellow wing-bars were visible on its crown usually contrasts less with the each wing: the greater-covert bar appeared mantle and, importantly, the lower mandible strong and of even width, extending across all is usually orange and the legs are dark. None visible feathers, and the median-covert bar of these possible confusion species share the was shorter but still conspicuous, often frequent tail-pumping habit of Pale-legged looking thinner distally. Leaf and Sakhalin Leaf Warblers. Morphologically, Pale-legged Leaf and Tail The uppertail was rarely visible in the Sakhalin Leaf Warblers are extremely similar field but did not appear to contrast with the and, based on current knowledge, cannot be mantle. reliably separated in the field. Even the bio- metric data of the two species overlaps, with Underparts Largely dull greyish-white, palest all but the largest Sakhalin Leaf and smallest on the throat, with a diffuse darker area often Pale-legged Leaf Warblers falling within an apparent on the breast-sides, close to the overlap zone. Based on a sample of 12 bend of the wing. Sakhalin Leaf and nine Pale-legged Leaf War- blers caught in Thailand for which the identi- Bare parts Bill dark with a conspicuous pale fication has been confirmed by DNA analysis, line along the cutting edge and, especially Round et al. (2016) considered that an indi- when viewed against the light, a striking vidual with a wing length longer than 65 mm small pale tip. The base of the underside of might reasonably be identifiable as Sakhalin the lower mandible was pinkish. Legs and Leaf Warbler. feet pale pink. For the time being, the formal separation of the two species relies on vocal and genetic Call Only heard at dusk, a repeated, single differences. Their songs are very different and short note (per PS). diagnostic, but the possibility of a singing bird occurring in a vagrancy context in Behaviour As active as the accompanying autumn is extremely unlikely. Vagrants do Chiffchaffs, constantly moving, gleaning from call, however, and the call frequency may be leaves and occasionally hovering. The bird con- diagnostic. Yap et al. (2014) made sound tinually tail-pumped – this dipping of the tail recordings of a presumed Sakhalin Leaf seemed slightly slower and more conspicuous Warbler that was wintering in Singapore and to the eye than the faster but less frequent tail compared these with online recordings of dips of the Chiffchaffs. The bird typically both species. They found that the call of remained within the mid-canopy of a single tall Pale-legged Leaf Warbler falls between 5.5– Sycamore, although it occasionally descended 6.4 kHz, whereas the call of Sakhalin Leaf into lower shrubs and bushes (per PS). Warbler is lower, within the range 4.4–4.7 kHz. The authors acknowledged, however, Identification that larger sample sizes are required, in par- As a species pair, Pale-legged Leaf and ticular for Sakhalin Leaf Warbler, to establish Sakhalin Leaf Warblers should be rather whether the frequency ranges of the calls are readily distinguishable from all other Phyllo- consistently different and do not overlap.

British Birds 111 • August 2018 • 438– 445 443 Headon et al.

More recently, Hungnon et al. (2017) demon- that of Eastern Crowned Warbler – hitherto strated that there is no overlap in the fre- the with the most easterly distribu- quency of the call notes of birds released after tion of any that has reached Britain (Holden ringing. From a sample of 33 individuals & Newsome 2011) – that species breeds as far trapped for ringing in Thailand, 19 Sakhalin west as the upper Argun River in southeast Leaf Warblers and 14 Pale-legged Leaf War- Transbaikalia, some 800 km to the west of the blers, the identities of which were confirmed known westernmost limit of Pale-legged Leaf by DNA analysis, they were able to confirm a Warbler’s distribution. difference in the frequency and duration of call notes, Sakhalin Leaf Warbler having a Vagrancy frequency of 4786.58 ± 191.08 Hz and dura- Although once thought improbable, it has tion of 0.084 ± 0.017 seconds, and Pale- become apparent in recent years that passer- legged Leaf Warbler having a higher ines breeding no closer than the Lake Baikal frequency call of 5729.29 ± 77.23 Hz and watershed are capable of reaching western duration of 0.066 ± 0.012 seconds. Europe. With this occurrence it is now pos- sible to push the extent of vagrancy potential back almost to the eastern seaboard of the Based on differences in song, Portenko Asian landmass – or even beyond to the (1950) proposed the recognition of P. bore- neighbouring islands. In terms of distance, aloides as a species distinct from P. tenellipes the core breeding grounds of Pale-legged Leaf but, owing to its publication in Russian, this Warbler and Sakhalin Leaf Warbler lie 7,400– went largely unnoticed in Europe. Martens 8,500 km and 8,300–9,100 km respectively (1988) identified significant differences in from southwest England (Paul Leader pers. DNA between the two taxa, and Weprincew comm.). Physiologically, Phylloscopus war- et al. (1989) noted habitat differences on the blers are certainly capable of travelling such breeding grounds that supported Portenko’s distances; for example, Willow Warblers P. findings. More recently, Kerr et al. (2009) trochilus of the race P. t. yukutensis migrate found the mitochondrial DNA sequence between Chukotka in northeastern Siberia divergence between the two taxa to be sub- and southern Africa, a distance of some stantial – COI gene: 3.7% uncorrected. Most 15,000 km or more each way. taxonomic authorities now recognise the two That a perhaps unique set of circum- taxa as distinct species, including the IOC stances resulted in an exceptional arrival of (Gill & Donsker 2018). east Asian vagrants at Portland at this time is not in doubt: quite apart from this first Distribution record for the Western Palearctic, subsequent Pale-legged Leaf Warbler breeds in a narrow days saw the discovery of Britain’s first zone of continental east Asia from the Amur Stejneger’s Stonechat Saxicola stejnegeri River southeast to northeast China and (Cade & Collinson 2015) and an ‘Isabelline northern Korea, whereas Sakhalin Leaf Shrike’ Lanius isabellinus/phoenicuroides also Warbler breeds only on the adjacent islands make landfall on the island. of Sakhalin, the Kuril Islands and northern Acknowledgments Japan. Both species winter in southeast Asia On behalf of all who took advantage of their generosity, but, owing to identification difficulties, their I would like to thank Pete and Debby Saunders for respective distributions during that season allowing access to their private garden; Pete also kindly and on migration have yet to be fully estab- provided the photographs that illustrate this paper. Paul Leader and Grahame Walbridge provided helpful lished (Yap 2014; Round et al. 2016). comments and encouragement throughout the Although their breeding ranges overlap with identification process. Martin Cade, Portland Bird Observatory, Portland, Dorset; e-mail [email protected]

References Brit. Birds 108: 423–428. Brazil, M. 2009. Birds of East Asia. Helm, London. Carey, G. J., et al. 2001. The Avifauna of Hong Kong. Cade, M., & Collinson, J. M. 2015. ‘Stejneger’s Hong Kong Birdwatching Society, Hong Kong. Stonechat’ in Dorset: new to Britain. del Hoyo, J., Elliott, A., & Christie, D. 2006. Handbook

444 British Birds 111 • August 2018 • 438– 445 Pale-legged Leaf Warbler: new to Britain

of the Birds of the World. Vol. 11. Lynx Edicions, Leader, P. J. 1993. The field identification of Arctic, Barcelona. Eastern Crowned, Two-barred Greenish and Pale- Fridolfsson, A. K., & Ellegren, H. 1999. A simple and legged Leaf Warbler in Hong Kong. Hong Kong Bird universal method for molecular sexing of non-ratite Report 1992: 153–159. birds. J. Avian Biol. 30: 116–121. Martens, J. 1988. Phylloscopus borealoides Portenko: Gill, F., & Donsker, D. (eds.) 2018. IOC World Bird List ein verkannter Laubsänger der Ost-Paläarktis. (v 8.1). doi: 10.14344/IOC.ML.8.1. J. Ornithol. 129: 343–351. Griffiths, R., Double, M. C., Orr, K., & Dawson, R. J. G. Portenko, L. A. 1950. [Reports on new and rare birds 1998. A DNA test to sex most birds. from the southeast area of the Soviet Union]. Mol. Ecol. 7: 1071–1075. Doklady Akad. Nauk SSSR, Novaya Seriya 70: 319– Hebert, P. D. N., Stoeckle, M. Y., Zemlak, T. S., & Francis, 322. (In Russian) C. M. 2004. Identification of birds through DNA Round, P. D., Pierce, A. J., Saitoh, T., & Shigeta, Y. 2016. barcodes. PLoS Biology 2: e312. Addition of Kamchatka Leaf Warbler Phylloscopus Helbig, A. J., Seibold, I., Martens, J., & Wink, M. 1995. examinandus and Sakhalin Leaf Warbler P. Genetic differentiation and phylogenetic borealoides to Thailand’s Avifauna. Bull. Japan Bird relationships of Bonelli’s Warbler Phylloscopus bonelli Banding Assoc. 28: 9–21. and P. nitidus. J. Avian Biol. 26: 139– Ticehurst, C. B. 1938. A Systematic Review of the 153. Phylloscopus. British Museum, London. Holden, D., & Newsome, M. 2011. Eastern Crowned Weprincew, B. N., Leonowitsch, W. W., & Netschajew, Warbler in Co. Durham: new to Britain. W. A. 1989. Zur Lebensweise von Phylloscopus Brit. Birds 104: 303–311. borealoides Portenko und Phylloscopus tenellipes Hungnon, C., Khudamrongsawat, J., Manawattana, S., Swinhoe. Mitteilungen aus dem Zoologischen Pierce, A. J., & Round, P. D. 2017. Distinguishing Museum in Berlin 65 (Suppl.): 71–80. between Sakhalin Leaf Warbler Phylloscopus Williamson, K. 1976. Identification for Ringers 2: borealoides and Pale-legged Leaf Warbler the genus Phylloscopus. BTO Field Guide No. 8. Phylloscopus tenellipes on call. Forktail 33: 77– 80. BTO, Tring. Kerr, K. C. R., Birks, S. M., Kalyakin, M. V., Red’kin, Y. A., Yap, F., et al. 2014. First wintering record of the Koblik, E. A., & Hebert, P. D. N. 2009. Filling the gap Sakhalin Leaf Warbler Phylloscopus borealoides in – COI barcode resolution in eastern Palearctic South-east Asia, with notes on vocalisations. birds. Front. Zool. 6: 29. Birding ASIA 21: 76–81.

Editorial comment Andrew Harrop, Chairman of BOURC, commented: ‘Observers involved with the 2012 Portland record must feel disappointed that, despite being seen quite well, pho- tographed and heard to call, such an exciting bird cannot be identified to species on current knowledge. Fortunately, we have additional biometric and genetic data for the 2016 record, and the latter proved conclusive in establishing its identification. There was no reason to doubt that it was a genuine vagrant. ‘This is an appropriate place for me to acknowledge the work done on DNA-based identifica- tions by Martin Collinson, my predecessor as Chair of BOURC. His work will be familiar to readers, has been a vital part of the assessment of some previous first records (e.g. Brit. Birds 110: 16–18) as well as this one, and has added significantly to our understanding of the status of several tricky taxa. ‘Having said that, birders should not feel disenfranchised. Although field identification of the species in this pair remains an unsolved challenge, the same was true a generation ago of several species which are now routinely identified in the field. Observers are encouraged to help find the key. For example, does Sakhalin Leaf Warbler tend to have a slightly smoother and more uniform grey crown and slightly less strikingly pale legs?’ Paul French, Chairman of BBRC, added: ‘The identification problems of this species pair are outlined above, and even though Pale-legged Leaf Warbler is now firmly on the British List, the possibility that Sakhalin Leaf Warbler will occur is very real and must be borne in mind with any future record. Any further claims of Pale-legged Leaf Warbler will need to demonstrate that Sakhalin Leaf Warbler can be ruled out, and ideally that will involve a DNA sample. However, extensive notes and/or photographs, and a sound recording (not just a description of the call) may also suffice for acceptance as our knowledge of their calls increases. This extraordinary couple of records proves that we really must have our eyes wide open when it comes to the possi- bilities for future eastern vagrants. The real questions lie in ‘why now?’ Why are we experiencing an upsurge in eastern at this point in time, and why are we getting them from so far away? Only time will tell if this pattern continues.’

British Birds 111 • August 2018 • 438– 445 445