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Britain's First Two-Barred Crossbill

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Britain's First Two-Barred Crossbill Britain’s first Two-barred Crossbill Andrew H. J. Harrop, Alan G. Knox and Robert Y. McGowan, on behalf of the British Ornithologists’ Union Records Committee Introduction isations that suggest that the three forms would Until 1998, the British Ornithologists’ Union be best treated as separate species (Benkman (BOU) compiled a single List of the birds of 1992; Elmberg 1993; Cramp & Perrins 1994; Britain & Ireland. Following a request from the AOU 1998). These differences are sufficiently Irish Rare Birds Committee, the BOU has since marked to be observable in the field. The His- then maintained a List for Britain alone (BOU paniolan Crossbill, now treated as a separate 1999). This has entailed ongoing work to sepa- species by the AOU (2003), is not discussed rate the two Lists and establish, among other here; for further information see Benkman things, the first acceptable records for Britain of (1994), Smith (1997) and Boon et al. (2006). those species and subspecies whose first British As a species, Loxia leucoptera was given its and Irish record came from Ireland. A number formal scientific name by Gmelin in 1789, of these were listed in recent BOURC reports in based on specimens of the American White- Ibis and by Bradshaw (2003). winged Crossbill brought to Europe from The first Two-barred Crossbill Loxia leu- Hudson Bay and New York. Therefore, early ref- coptera for Britain & Ireland was a female of the erences to this bird in Britain were generally Old World race L. l. bifasciata (hereafter under the name ‘White-winged Crossbill’ and referred to as bifasciata), shot at Grenville, near believed to relate to birds from America (e.g. Belfast, Co. Antrim, on 11th January 1802 Pennant 1812, Fleming 1828, Jenyns 1835). In (Thompson 1849). Several of the early records 1827, Brehm described the form currently were noted as having been ‘White-winged regarded as the Old World race as a separate Crossbills’, the name by which we currently species – L. bifasciata – and the two birds know the North American nominate race L. l. became known as the ‘American White-winged leucoptera (hereafter referred to as leucoptera). Crossbill’ and the ‘European White-winged This subspecies could conceivably occur as a Crossbill’. Specimens of both forms remained vagrant in Britain so, when seeking to establish quite rare in museums and some ornithologists the first British record of Two-barred Crossbill, maintained that the two were one and the same it was also necessary to determine which sub- until 1842, when their characters were clarified species had occurred here. (see Yarrell 1839–41, 1856). A certain amount of confusion remained, however, and several nine- Diagnosability and taxonomy teenth-century records from Britain were Three forms of Two-barred Crossbill are cur- attributed to ‘American White-winged Crossbill’ rently recognised: bifasciata (Two-barred Cross- by authors apparently unaware of the distinc- bill) in Eurasia, leucoptera (White-winged tion. Although the name ‘Two-barred Crossbill’ Crossbill) in northern North America, and was proposed in 1848 (Newman 1848), it was megaplaga (Hispaniolan Crossbill) in the not until much later that it entered general use mountains of Hispaniola. Although geograph- (e.g. Newton 1882). ical variation across these taxa has been described as ‘rather slight’ (Cramp & Perrins Measurements and structure 1994), there are clear and consistent differences Overall, leucoptera is a considerably smaller and in measurements, structure, plumage and vocal- lighter bird than bifasciata, with a proportion- 650 © British Birds 100 • November 2007 • 650–657 Britain’s first Two-barred Crossbill (e.g. Newton 1882, Cramp & Perrins 1994). On leucoptera, the scapulars, wing and tail appear blacker, and the pale fringes of tail- and flight- feathers are narrower; the white tips of the greater coverts and especially the tertials (though normally still striking and conspic- uous) are narrower (sometimes appreciably so); and males exhibit three main colour morphs – dark crimson, pastel pink and orange-pink – of which the first tends to be a slightly deeper, Ian Lewington more rosy-crimson shade of red than any bifas- Fig. 1. These drawings highlight the different bill ciata. There are other differences too: on structure of Two-barred Crossbill Loxia leucoptera bifasciata (left) and White-winged Crossbill average, female leucoptera appears greyer and L. l. leucoptera (right). Note that bills have been darker than female bifasciata, and darker represented crossing the same way to facilitate feather centres create more prominent crown, comparison. In life there is bill dimorphism, mantle and breast markings; and both sexes of although most have the tip of the lower leucoptera tend to be more heavily marked on mandible on the right side. the underparts, especially on the flanks and ately smaller head and an almost equally long, undertail-coverts. When applying these criteria but much more slender, bill (table 1). These dif- it is important to try to age and sex individual ferences in size and structure are noticeable in birds to ensure that comparisons are valid. the field, though the shorter mean wing length of leucoptera is difficult to assess without in- Vocalisations hand confirmation. As noted by Boon et al. (2006), bifasciata and The bill of bifasciata is heavy and often leucoptera both have relatively uniform vocal indistinguishable from that of Common Cross- repertoires across their extensive breeding bill L. curvirostra. The bill of leucoptera, as well ranges, and these vocalisations are diagnosably as being much less deep and wide, differs in distinct. Elmberg (1993) described the song of shape from that of bifasciata. Typically, the tip bifasciata as rich and varied, including metallic of the upper mandible in leucoptera extends and slurred whistles but also chatters and noticeably further beyond the tip of the lower wheezes, interspersed with any of the three basic mandible. In the hand, when viewed from flight-calls. It is thus reminiscent of the song of directly in front, the line of the culmen of bifas- Common Crossbill. In comparison, the song of ciata is fairly straight from left to right whereas leucoptera is more monotonous, including long in leucoptera the slender tip of the upper trills on different pitches, recalling the song of mandible is (diagnostically) deflected laterally Greenfinch Carduelis chloris. towards the side on which the tip of the lower The calls of bifasciata include a high-pitched mandible occurs (see fig. 1). ‘glip glip’ and the nasal so-called ‘trumpet-call’, for neither of which leucoptera has a close Plumage equivalent call. Conversely, the calls of leu- Some plumage differences between bifasciata coptera include a ‘keck’ unlike any call of bifas- and leucoptera have been described previously ciata (Cramp & Perrins 1994). For a detailed Table 1. Measurements of Two-barred (Loxia leucoptera bifasciata) and White-winged (L. l. leucoptera) Crossbills (data from Benkman 1992, Cramp & Perrins 1994). All measurements taken from skins except weight, which is taken from live specimens in Russia (bifasciata) and Canada (leucoptera). Bill length measured to skull. Measurements are given as a range (sample size in parentheses) except weight of leucoptera, which is given as mean ± standard deviation. Male bifasciata Male leucoptera Female bifasciata Female leucoptera Bill length (mm) 19.4–22.4 (21) 18.0–21.2 (8) 19.3–22.4 (18) 17.4–19.4 (10) Bill depth (mm) 9.7–11.0 (18) 7.8–8.8 (8) 9.4–10.7 (14) 7.6–9.0 (10) Bill width (mm) 8.7–10.2 (10) 7.5–8.2 (5) 8.3–9.5 (7) 7.0–8.0 (7) Wing length (mm) 88–96 (23) 88–93 (8) 87–94 (18) 83–88 (11) Weight (g) 26.3–40.0 (35) 26.0±1.27 (119) 24.9–35.3 (15) 24.4±1.52 (100) British Birds 100 • November 2007 • 650–657 651 Britain’s first Two-barred Crossbill adult male bifasciata adult male adult male orange-pink dark crimson leucoptera leucoptera adult male pastel pink leucoptera 1st-winter female bifasciata 1st-winter female leucoptera adult female bifasciata adult female leucoptera Ian Lewington Fig. 2. Plumages of Two-barred Crossbill Loxia leucoptera bifasciata and White-winged Crossbill L. l. leucoptera. account of the vocalisations of bifasciata,see Likelihood of vagrancy by bifasciata and Ebels et al. (1999). Differences in vocalisations leucoptera between the two taxa have also been described In the Old World, bifasciata breeds widely and illustrated by Constantine & The Sound throughout northern boreal forests, from Approach (2006). Finland and western Russia eastwards. It occurs 652 British Birds 100 • November 2007 • 650–657 Britain’s first Two-barred Crossbill regularly farther south, with irruptive movements 25 bringing it to Britain and other parts of western 20 Europe, apparently on a seven-year cycle in line with 15 a similar cycle of Larch Larix seed crops in western Russia 10 (Larsen & Tombre 1989). It is likely that irruptions 5 occur when a good seed crop in the previous year has 0 led to a successful breeding 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 season which is then fol- Fig. 3. Accepted records of Two-barred Crossbill Loxia leucoptera in Britain, lowed by a poorer seed year, 1982–2006. forcing large numbers of birds to move in search of food. The seven-year cycle is not a reliable predictor of vagrancy, however, since bifasciata occurred in Britain in variable numbers in 15 of the 25 years between 1982 and 2006, with peaks in 1987, 1990 and 2002 (fig. 3). In North America, leu- coptera breeds in northern coniferous forests from © Natural History Museum,Tring Newfoundland to Alaska. It winters mainly within boreal forests, and under- takes irregular movements Harrop Andrew in response to food avail- ability.
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