The Scottish Crossbill – what we know and what we don’t Ron W. Summers and Stuart B. Piertney Alan Harris ABSTRACT The Scottish Crossbill Loxia scotica is Britain’s only endemic bird. Given this status, together with its small population size and its assumed association with the few remaining scraps of native pinewood, the Scottish Crossbill has the highest of conservation designations. Its biology is poorly known, however, making it difficult to implement conservation measures. Here, we review some recent work which is investigating different aspects of the biology of crossbills in general and the Scottish Crossbill in particular. Museum, field and laboratory-based studies examining biometrics, vocalisations, feeding ecology, movements and genetics have now clarified some of the inter- relationships between the three crossbill species which are now recognised as breeding in Britain. Such efforts are also helping to identify the future research priorities required to maintain populations. 100 © British Birds 96 • March 2003 • 100-111 The Scottish Crossbill – what we know and what we don’t he Scottish Crossbill Loxia scotica is one so potential hybridisation between Scottish and of Britain’s most charismatic and, at the Parrot Crossbills was not regarded as an issue. Tsame time, one of its most enigmatic It is thought that the Scottish Crossbill is a birds: charismatic in that it is designated as relict species derived from a crossbill popula- Britain’s only avian endemic species and so is a tion which was isolated in the Caledonian pine flagship for natural heritage; but enigmatic in forests of Scotland when Britain became sepa- that there are still considerable gaps in our rated from continental Europe after the last knowledge about fundamental aspects of the glaciation (Nethersole-Thompson 1975; biology and ecology of crossbills in general and Murray 1978; Voous 1978). During this period Scottish Crossbills in particular. of isolation, the Scottish Crossbill could have Recent and on-going work by the RSPB, the evolved its unique intermediate size characteris- University of Aberdeen, the Centre for Ecology tics from an ancestral stock of either Common and Hydrology, Forest Enterprise and other or Parrot Crossbills in response to the available crossbill enthusiasts has meant that our under- Scots Pine Pinus sylvestris cones (tougher than standing of the biology of the Scottish Crossbill spruce Picea cones, yet less likely to be frozen) is improving. The aim of this paper is to review (Knox 1975; 1990a). An alternative hypothesis some of the findings of this work, and put them is that the Scottish Crossbill has remained into context with the problems associated with similar to an ancestral stock of Parrot Cross- the conservation of the Scottish Crossbill. bills, and that Parrot Crossbills in mainland The Scottish Crossbill has had a somewhat Europe increased in size when Common Cross- turbulent taxonomic history. It was initially bills began to invade from the east and breed described as a subspecies of the Common sympatrically with Parrot Crossbills (Nethersole- Crossbill L. curvirostra, in 1904, because it is Thompson 1975). This process is known as intermediate in size between Common and ‘character displacement’ (Mayr 1963). Parrot Crossbills L. pytyopsittacus. The type- The Caledonian pine forest, which once specimen was collected on 26th December 1870 covered much of Highland Scotland, has been in Ross-shire and named L. c. scotica (Hartert reduced in size over the past 5,000 years as a 1904). There is, however, overlap in wing length result of forest clearance by farmers and and bill length measurements between Scottish perhaps climate change (Smout 1993; Tipping Crossbill and both Common and Parrot Cross- 1994). Consequently, less than 1% of the Cale- bills (Knox 1976), and there are no diagnostic donian forest remains and occurs only as scat- plumage differences. Given its intermediate size, tered fragments, primarily in Deeside and there were also calls to classify Scottish Crossbill Strathspey. The largest fragment is Abernethy as a subspecies of Parrot Crossbill (see Knox Forest. With its endemic species status, sup- 1975 for a review), but this convention was not posed small population size and restricted widely accepted (Witherby et al. 1943). ancestral habitat, the Scottish Crossbill has been The Scottish Crossbill L. scotica is currently given the highest categories of conservation regarded as a full species by the BOU, and con- importance: it is of global conservation concern sequently is Britain’s only endemic bird (Voous (as a result of data deficiency); a Species of 1977; BOURC 1980). Its designation as a species European Conservation Concern Category 1; was based on the observation that Scottish and it is on the ‘Red list’ of Birds of Conserva- Crossbills maintained their intermediate size tion Concern in the UK (Tucker & Heath 1994; between Common and Parrot Crossbills, despite Gregory et al. 2002). Clearly, there is a need for repeated invasions by Common Crossbills from a conservation programme to safeguard this the Continent (Knox 1976). Furthermore, Scot- species, and a UK Biodiversity Action Plan was tish and Common Crossbills were found to drawn up for the Scottish Crossbill (Anon breed sympatrically in Deeside, but they appar- 1995). A fundamental problem with the Scot- ently did not hybridise (Knox 1990a,b). Conse- tish Crossbill, however, is the difficulty of iden- quently, Scottish Crossbills satisfied the tifying it in the field, because it is so similar in Biological Species Concept (Mayr 1963) that appearance to both Common and Parrot Cross- species are ‘groups of interbreeding natural pop- bills (Knox 1990c). This has led to uncertainty ulations that are reproductively isolated from about its range within Britain (Knox, in other such groups’. At this stage, Parrot Cross- Gibbons et al. 1993), its population size bills were not known to be breeding in Scotland, (Nethersole-Thompson 1975) and its habitat British Birds 96 • March 2003 • 100-111 101 The Scottish Crossbill – what we know and what we don’t requirements. These gaps in our knowledge stimulated the initia- tion of several studies in the early 1990s. As well as studying range and habitats, the relation- ships among Scottish, Common and Parrot Crossbills were inves- tigated in terms of biometrics, vocalisations and genetics (Piertney et al. 2001; Marquiss & Rae 2002; Summers et al. 2002). Can Scottish Crossbills be identified by their bill size? One of the recent studies started Ron W. Summers with a re-examination of 64. Male Parrot Crossbill Loxia pytyopsittacus, Abernethy Forest, museum specimens. Initially, the Highland, April 2001. biometrics of Parrot and Common Crossbills from Fennoscandia were described. There were no intermediate bill sizes in the sample from Fennoscandia, so there was no doubt about species identifica- tion in these populations. These formed the baseline against which birds from Scotland were compared. A comparison with museum specimens collected in Scotland showed that there were Common Crossbills and birds of intermediate size (i.e. Scottish Crossbills) in the Highlands of Ron W. Summers Scotland. The data from these 65. Female Scottish Crossbill Loxia scotica, Glen More, two groups did not, however, Highland, April 2002. separate into distinct clusters (Summers et al. 2002), so it was impossible to be sure of the identity of birds lying within the overlap zone. This meant that some birds could not be identi- fied with certainty by size. In the sample of museum specimens from Scotland, there were also some birds which could have been classed as Parrot Crossbills. Because they occurred at the lower end of the size distribu- tion of Parrot Crossbills, however, it is possible that they were, in fact, Scottish Crossbills. Generally, most collecting of Ron W. Summers 66. Male Common Crossbill Loxia curvirostra, Abernethy Forest, crossbills occurred during the Highland, April 2000. late nineteenth century and early 102 British Birds 96 • March 2003 • 100-111 The Scottish Crossbill – what we know and what we don’t 14 Male 14 Female 13 13 12 12 11 11 Bill depth (mm) Bill depth (mm) 10 10 9 9 1850 1890 1930 1970 2010 1850 1890 1930 1970 2010 Fig. 1. The bill depths of museum specimens and live-trapped crossbills Loxia in Scotland in different years. All measurements are by RWS.The lower limits for the 95% range for Parrot Crossbills L. pytyopsittacus are 12.5 mm for males and 12.3 mm for females and the upper limits for Common Crossbills L. curvirostra are 11.3 mm for males and 11.2 mm for females.The years with documented Parrot Crossbill invasions are shown by triangles. part of the twentieth century, with relatively few crossbills, but these differences were not for- specimens from the late twentieth century. mally described. In order to investigate the calls Nonetheless, it was assumed that the museum of crossbills, we made tape recordings and specimens were a typical sample of birds living sonograms of birds of known bill size. Fol- in Scotland. It was, therefore, a surprise when we lowing the work of Groth (1993) on North started catching crossbills in the Mar Lodge woods and in Abernethy Forest in the 1990s to find that many of the birds were Parrot Cross- bills (fig. 1) (Marquiss & Rae 2002; Summers 2002). It is possible that these birds originated A B E from recent invasions which took place in 1962, 1982 and 1990 (Thom 1986; Jardine 1992). Common Crossbill Parrot Crossbills are known to undertake spo- radic eruptions from their Fennoscandian and Russian breeding grounds, although these are not as frequent as in the Common Crossbill (Newton 1972). Parrot Crossbills feed primarily C on the seeds from Scots Pine cones, which are generally available annually, thus reducing the Scottish Crossbill need to emigrate. This is in contrast to the Norway Spruce Picea abies, which produces cones more erratically, leading to the larger and more frequent eruptions of Common Crossbills, which rely on this conifer in their north Euro- pean breeding grounds.