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IDENTIFICATION OF BONELLI’S WARBLERS

Eastern Bonelli’s Warbler Phylloscopus orientalis (Ian Lewington) IDENTIFICATION OF BONELLI’S WARBLERS

DOUG PAGE ILLUSTRATED BY IAN LEWINGTON

he recent decision by the British recorded in Britain and Ireland, of which Ornithologists’ Union Records Com- 56 have been accepted as bonelli and just Tmittee to treat Bonelli’s Warbler three specifically assigned to orientalis. The Phylloscopus bonelli as two , Western paucity of records attributable to orientalis Bonelli’s Warbler Phylloscopus bonelli and clearly reflects the respective population Eastern Bonelli’s Warbler Phylloscopus ori- strengths and the north-south migration entalis (Ibis 139: 197-201; Brit. 90: 70; route of orientalis, which seems less likely 91: 122-123), is a potential cause of frustra- to overshoot in spring, but a clear under- tion for the observer who has the good standing and awareness of the field fortune to find a vagrant, silent ‘Bonelli’s characters is also perhaps a contributory warbler’. The two species are very similar, factor. both structurally and in plumage features, It should be noted that, although all but thankfully the calls are dissimilar and, three British records have occurred in in the case of orientalis, highly distinctive. autumn, of four accepted Dutch records up The purpose of this short paper is to high- to 1995, three were in spring and one was light the known identification features and in July. to encourage observers to try to determine additional field characters of the two species. DISTRIBUTION AND Current knowledge suggests that the WINTERING AREAS majority of silent individuals will not be The Western form, bonelli, has a breeding readily identifiable in the field, but, with range extending from Northwest , careful observation, at least some individ- north through Iberia and , through uals may be specifically identified. Of Central , Italy and east to Austria course, if one is calling, the identification and northwestern former Yugoslavia. should be straightforward. Numerically, the population is large, with Up to the end of 1998, there has been a an estimate in the EBBC Atlas of 1.5-4 combined total of 145 Bonelli’s warblers million breeding pairs. The wintering area

524 British Birds 92: 524-531, October 1999 IDENTIFICATION OF BONELLI’S WARBLERS

Figure 1. Eastern Bonelli’s Warbler Phylloscopus orientalis and Western Bonelli’s Warbler P.bonelli, and pit- falls: the Siberian race of P.collybita tristis and the nominate race of Booted Warbler Hippolais caligata (Ian Lewington) is along the southern edge of the Sahara, and early May. from Senegal in the west to northeast The eastern form, orientalis, has a much Nigeria and northern Cameroon in the smaller population, occurring in central east, roughly between 10° and 17°N. Given and southern former Yugoslavia, northern the West African wintering range, the main , Greece and western , with migration route is south to southwest in small numbers in Lebanon and Syria. The autumn, with the peak southerly move- estimated population given in the EBBC ment occurring in August. Spring Atlas is only 15,000-40,000 breeding pairs, migration is from early March to which represents just one percent of the the middle of May, with the peak in April number of bonelli. The complete extent of

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Table 1. Plumage of Eastern Phylloscopus orientalis and Western Bonelli’s Warblers P.bonelli. Observers should note that there is a considerable degree of overlap between the two species. First-winter individuals of both species appear ‘cleaner’ and brighter than autumn adults, which have worn and bleached remiges and rectrices. Adults of bonelli, and presumably orientalis, tend to show a more contrasting rump-patch than do first-winters, which can be duller, showing less contrast.

Feature orientalis bonelli Supercilium Whitish, quite prominent, Whitish, but often tinged with probably never showing yellow or buff and, therefore, any yellow tones less prominent Lores Pale, but the dark smudge in Pale, the dark smudge in front front of the eye tends to be of the eye is rather diffuse and more obvious less obvious Ear-coverts Pale brown to warm almost Pale brown. Less contrast with gingery-brown, tending to more brownish head tones contrast with greyish head tones Upperparts Cold, greenish-grey, lacking Less grey, more brownish- brown tones, often looking olive or pale brownish (can be very grey with just a slight quite similar in tone to Booted olive or green tone to the Warbler Hippolais caligata) lower mantle Fringes to remiges Duller greenish-lime Brighter yellowish-green and rectrices Tertial fringes More contrast with Less contrast, with greyer greenish-edged remiges, with tertial edgings whitish or silver-white edgings Axillaries and Pale yellow or whitish Brighter yellow underwing-coverts Uppertail-coverts More contrast between the Less contrast between yellow yellow rump and dingy, rump and yellow tones of longer uppertail-coverts longer uppertail-coverts Bill Tends to show more- Extensive pale pink lower restricted pale base to lower mandible, upper mandible mandible horn with pinkish cutting edge and base. Whole bill often gives impression of being pink Legs Dark grey-brown or blackish- On average, paler, often with brown pinkish-brown tone

the winter quarters is as yet unknown, but FIELD CHARACTERS (table 1, fig. 1) migration is basically north-south, with at Both species are medium-sized Phyllo- least part of the population wintering in scopus warblers, slightly smaller than Sudan south to about 9°N. Migrants have P. trochilus. Although size reportedly occurred in Malta, Tunisia and is of little value in the field, orientalis aver- western Libya, so it is possible that part of ages slightly larger than bonelli. Both the winter quarters lie farther to the west species display a character similar to that than is currently known. Timing of migra- of P. sibilatrix, but appear tion is similar to that of bonelli, but spring proportionately longer tailed and are migration starts earlier, in late February. shorter winged than that species. Primary

526 British Birds 92: 524-531, October 1999 IDENTIFICATION OF BONELLI’S WARBLERS

190. Eastern Bonelli’s Warbler Phylloscopus orien- 191. Eastern Bonelli’s Warbler Phylloscopus orien- talis, Eilat, Israel, April 1987 (Hadoram Shirihai). talis, Eilat, Israel, April 1986 (Hadoram Shirihai). A rather warm-coloured, yellower/greener indi- A paler/greyer individual than that in plate 190. vidual, approaching Western P. bonelli in coloration. projection is of medium length, falling bright golden-yellowish patch can be seen between those of Willow Warbler and on the rump, although this is dull on first- Common Chiffchaff P. collybita. In addition, winters and on a few spring birds. the nasal hairs are more pronounced than Underparts are a clean, chalky-white, often those of trochilus, with the three rictal bris- described as silky-white, with, occasion- tles overhanging the base of the nostrils ally, a pale-buff, greyish or yellow wash to (Williamson 1962). the breast sides and/or yellow streaking Plumage is rather plain with, for a Phyl- on undertail-coverts. loscopus, a rather bland facial expression, The bill is relatively long, and of owing to a lack of obvious loral or eye medium strength, fleshy-orange, with dark stripes. A pale supercilium is rather culmen and tip, and can appear remark- diffuse, and a rather large, dark eye, accen- ably pale when viewed from below. Legs tuated by a complete pale eye-ring, stands and feet, although variable, are generally out in the pale face. Mantle and scapulars grey-brown or blackish-brown, or pinkish- are plain greyish-brown or brownish-grey, brown. with greenish or olive tones; tertials are Differences between the two species are, dark-centred, with paler, ‘faded’ fringes; at best, subtle, but on some individuals a and the wing-coverts, remiges and rectrices combination of characters may be present are edged lime or yellowish-green. When which helps to afford a specific identifica- wing-flicking or -catching, a small, tion (see below and table 1).

192. Two Eastern Bonelli’s Warblers Phylloscopus 193. Eastern Bonelli’s Warbler Phylloscopus orien- orientalis, Eilat, Israel, April 1986 (Hadoram Shir- talis, Sumburgh Hotel and Grutness, Shetland, ihai). Showing individual variation in size and August 1998 (Bill Jackson) coloration.

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194-196. Western Bonelli’s Warbler Phylloscopus bonelli, Sumburgh Head, Shetland, September 1995 (Bill Jackson)

those of orientalis are colder grey, lacking the brown tones of bonelli. The latter can, however, also appear very pale greyish- brown, often with an olive-brown tone (such pale, grey-brown-mantled individ- uals have, surprisingly, on more than one occasion in autumn been misidentified as Eastern orientalis has a ‘sharper’, better- Booted Warbler Hippolais caligata). This, defined head pattern owing to a whitish coupled with its grey head, makes orientalis supercilium, which on Western bonelli a colder, altogether greyer-looking species. tends to be tinged with buff or yellowish. On orientalis, the fringes of the remiges and Also, although the lores of both species are rectrices are dull greenish-lime, whereas on pale, there is a small dark smudge just in bonelli they are brighter, yellowish-green, front of the eye and this tends to be more forming a more obvious panel on the obvious on orientalis, but more diffuse on closed wing. Although there is much bonelli. The head colour is greyer on orien- overlap, the dark-centred tertials of orien- talis, and the pale-brown or ginger-brown talis show a more contrasting pattern ear-coverts tend to contrast with the grey against the greenish-edged remiges, as the tones, whereas, with the head colour of actual fringes to the tertials are white, bonelli browner, the ear-coverts show less silvery-white or yellowish-white compared contrast. with the duller, greyer fringes on bonelli. Upperpart tones vary, but generally Although of limited use in the field, the

197 & 198. Western Bonelli’s Warbler Phylloscopus bonelli, Sumburgh Head, Shetland, September 1995 (L. Dalziel)

528 British Birds 92: 524-531, October 1999 IDENTIFICATION OF BONELLI’S WARBLERS axillaries and the underwing-coverts of ori- silent individuals will inevitably remain entalis are whitish or a pale yellow, as indeterminate. opposed to the rather bright yellow of bonelli. MOULT Worn adults of both species in Since the general upperpart tones are autumn are more likely to appear faded duller on orientalis, the yellow rump patch and bleached, with worn and faded may show more contrast or stand out more remiges and rectrices, than fresh first- clearly than does that of bonelli. winter individuals. Ageing, especially of As with plumage features, there is con- those in autumn, although not a prerequi- siderable overlap between the coloration of site to acceptance, would certainly help in the two species’ bare-parts, but generally the identification process. those of bonelli average paler than those of Adults undergo a partial post-breeding orientalis. The bill of bonelli can look moult of the body feathers, and occasion- remarkably pale pink, especially when ally the tertials, on or near the breeding viewed from below, whereas that of orien- grounds in July-August. The pre-breeding talis tends to be darker, with a restricted moult, usually undertaken from mid pale base to the lower mandible. October onwards in the winter quarters, is Leg colour is generally dark on both complete, with primaries moulted descen- species, but that of bonelli can show a pink, dently. paler tone, particularly at the rear. Juveniles undertake a partial moult of Despite the above, observers should be the body only from July or August to Sep- aware that some of these features are sub- tember. jective and, at present, the majority of VOICE The song of both species is a short, loose shivering trill of an unvaried note; lower-pitched, slower, the notes more sepa- rated, and more musical than that of Wood Warbler and lacking that species’ accelera- tion and increased vibrancy. The character is reminiscent of Sylvia curruca or a distant Cirl Bunting Emberiza

199-201. Western Bonelli’s Warblers Phylloscopus bonelli,The Gambia, February 1996 (R. Riddington)

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202. Eastern/Western Bonelli’s Warbler Phylloscopus orientalis/bonelli, Exnaboe, Shetland, October 1992 (L. Dalziel)

Without doubt, the biggest single aid to identification and perhaps the only conclu- sive element is the call. That of bonelli is a rather typical Phylloscopus ‘poo-weet’, ‘hoo- eet’, ‘chweet’ or ‘clo-eee’, resembling that of Willow Warbler, but slower, shriller and clearly more disyllabic. The call of orientalis is totally different, being most un-Phyllo- scopus-like. It is an abrupt, sharp, quite loud, monosyllabic ‘chip’, ‘tchip’ or ‘khip’, recalling a distant, less incisive Common 203. Western Bonelli’s Warbler Phylloscopus bonelli, , June 1991 (Kevin Carlson) Loxia curvirostra or even a sparrow Passer. cirlus. Although the songs of the two The British records of orientalis have Bonelli’s warblers are extremely similar, involved particularly vocal individuals, that of orientalis is reputed to be shorter, less and it may be that orientalis does call more vigorous and less frequently uttered than often than bonelli, but this could be biased that of bonelli. It must be noted, however, by the particularly distinctive note of orien- that there is individual variation in the song talis. It is also obvious that silent orientalis of both species and identification on song will currently make up an unknown pro- alone, under field conditions, is inadvisable. portion of the 59% of British & Irish records not assigned to either species.

BIOMETRICS The two species are extremely similar in their measurements, and even when in the hand can be dif- ficult to separate. Although orientalis is

204. Western Bonelli’s Warbler Phylloscopus bonelli, , August 1997 (Roger Tidman)

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205 & 206. Western Bonelli’s Warblers Phylloscopus bonelli, Spain, May 1994 (Mike Lane) slightly larger than bonelli, with fractionally recognition problems, but, with time and longer wings, such differences are of little careful observation, it is hoped that some use in the field. Wing-formula differs in of the rather tentative criteria put forward that bonelli has the second primary usually here will be confirmed as distinct features just shorter than the sixth, whereas orien- which will help to make the identification talis has the second primary usually just of silent individuals more certain. longer than the sixth. Similarly, the sixth primary of bonelli is often emarginated, but ACKNOWLEDGMENTS that of orientalis is not clearly emarginated Lance Degman, Paul Harvey, Mike Rogers (although many individuals show at least a and Grahame Walbridge all provided hint of emargination). constructive criticism of the draft paper. Thanks are due to the photographers who CONCLUSIONS provided reference material, as well as the Observers should be aware that there is a 17 portraits included here: Dr Kevin wide range of overlap between the two Carlson, Larry Dalziel, Bill Jackson, Mike species so far as plumage, bare-part charac- Lane, Dr Roger Riddington, Hadoram teristics, wing-formula and biometrics are Shirihai and Roger Tidman. Special thanks concerned, and the utmost caution is there- are due to Ian Lewington for his superb fore needed when attempting to identify plate of Bonelli’s Warblers. specifically either species, even in the hand. REFERENCES The distinctive calls, of which there is no Cramp, S. 1992. The Birds of the Western Palearctic. known overlap, are by far the safest and vol. 6. Oxford. easiest way for an accurate identification to Hagemeijer, W. J. M., & Blair, M. J. 1997. The EBBC Atlas of European Breeding Birds. be made. . It should be noted that date and geo- Harris, A.,Tucker, L., & Vinicombe, K. 1989. The graphical location (as mooted in some Macmillan Field Guide to Identification. London. circles) are in no way an indication of Svensson, L. 1992. Identification Guide to European which species may be involved when . Stockholm. dealing with silent individuals. There are Williamson, K. 1962. The Phylloscopus. Identification for Ringers 2. Tring. several examples of late-autumn birds from the Northern Isles, the English east coast and the Isles of Scilly which have AUTHOR been proven to be bonelli. Doug Page, 11 Ashley Court, Finningley, It seems likely that this species pair will, Doncaster, South Yorkshire DN9 3RA for the foreseeable future, pose severe

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