Print 03/03 March 2003

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.

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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

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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

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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. The presence of Parrot D Crossbills in Scotland has therefore further com- Parrot Crossbill plicated the picture.

Can Scottish Crossbills be identified by Fig. 2. Sonogram outlines of excitement calls of crossbills Loxia found in Scotland. Examples can be their vocalisations? heard on the following website: Nethersole-Thompson (1975) noted that Scot- www.rspb.org.uk/wildlife/scisurv/speciesrsch/ tish Crossbills have different calls from other scotcroscalls.asp

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American crossbills, we concentrated on the Crossbills occurred mainly in the eastern High- single-note flight and excitement calls, which lands with outlying records in the Flow are given by both sexes. Unlike bill size, which Country of Caithness and Sutherland, showed no distinct clustering, the calls fell into Perthshire, Angus, Stirlingshire and Fife (fig. 3). distinct types, as seen on the sonograms. Five In contrast, Common Crossbills were much types of excitement call were recognised in more widespread across Scotland, while Parrot Scotland, and by matching these up with the bill Crossbills were found in only a few woods in size of the birds which gave them, the following Strathspey and Deeside (Marquiss & Rae 2002; patterns were apparent (fig. 2). Common Cross- Summers et al. 2002). The other important dis- bills gave three different types of excitement covery was that Scottish Crossbills occurred calls, Parrot Crossbills had only one type of call commonly in plantations as well as in ancient and the intermediate Scottish birds had their native pinewoods. These plantations either own excitement call (Summers et al. 2002). comprised largely Lodgepole Pine Pinus con- With each excitement call there was an associ- torta, Sitka Spruce Picea sitchensis, larches Larix ated flight call. It is likely that calls are learnt by spp. or Scots Pine, or mixtures of these species. young birds from their parents and so should Nonetheless, during January to March, cross- remain constant within a species. We do not bills in woods where Scots Pines were dominant know, however, whether crossbills can change or co-dominant were more likely to be Scottish their calls after becoming independent from than Common Crossbills (Summers et al. their parents, or what happens when crossbill 2002). species interbreed. Nevertheless, it does appear Another way to study the habitats of cross- that, at the moment, we can use these calls as a bills of known identity is to observe colour- means of identification. It must also be pointed ringed birds with known bill measurements. In out that other published sonograms of Scottish Deeside, Marquiss & Rae (2002) found that Crossbills must be regarded as suspect because crossbills with the largest bills (Parrot Cross- the calls were not matched to bill size. bills) fed on Scots Pines throughout the year and tended to be found in the native pinewoods Where are Scottish Crossbills found? (Glen Tanar, Ballochbuie and Mar Lodge The excitement call described above has allowed woods). Birds with the smallest bills (Common us to assess the range of the Scottish Crossbill. Crossbills) fed on Sitka Spruce and larches, and By making tape recordings throughout on Scots Pine in late spring when the cones northern Scotland, we found that Scottish opened and the seeds became readily available. Ron W. Summers 67. Native pinewood, Strathfarrar, Highland, August 2001.

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Birds with intermediate bills (Scottish Crossbills) fed mainly on larches and Scots Pines, and occasionally on Sitka Spruces. The findings of Marquiss & Rae are therefore similar to those of the wider- ranging study where birds were identified from sono- grams (Summers et al. 2002). Although there were ecolog- ical differences among the three groups, there was con- siderable overlap in habitat and diet. An analysis of habitat data for Common and Scottish Crossbills revealed a habitat overlap of 73% during January to March (Summers et al. 2002).

What are the movements of Scottish Crossbills? Crossbills are famous for their Fig. 3. The distribution of crossbills giving excitement call C (i.e. Scottish movements in response to Crossbills Loxia scotica). From Summers et al. (2002) and unpublished data. fluctuations in their conifer food supply (Newton 1972). This also applies to movements are local within Scotland. For crossbills in Scotland (Nethersole-Thompson example, one bird was recorded moving from 1975; Summers 1999). What little we know Deeside to Strathspey (Marquiss et al. 1995), about the movements of individual Scottish and a movement from Strathspey to Deeside Crossbills comes largely from colour-marked has also been noted. birds in Deeside. Movements of less than 18 km were noted, usually between woods with similar What is the niche of the Scottish Crossbill? conifer species (Scots Pines and larch). Mar- The general association between bill size and quiss & Rae (2002) observed that Scottish and food preference of crossbills (Lack 1944), plus Parrot Crossbills tended to be resighted in fol- feeding studies of captive crossbills (Benkman lowing years, i.e. they were resident. In contrast, 1987) provide evidence that the different none of the small-billed birds in Deeside were species have specialised niches. Even the names resighted in later years, reflecting the more of the crossbills allude to their specialisation. nomadic behaviour of Common Crossbills. For example, the Norwegian names for Only the small-billed birds were found to have Common and Parrot Crossbills are the ‘Spruce stores of premigratory fat, indicating prepara- Crossbill’ (Grankorsnebb) and ‘Pine Crossbill’ tion to depart, usually in late spring and early (Furukorsnebb), respectively. The Scottish summer. Spruce crops are well known for their Crossbill does not, however, fit easily into this erratic coning, leading to boom (‘mast years’) pattern because, along with the Parrot Crossbill, and bust years. Scots Pine also shows annual it is thought to be adapted to feeding on Scots variations in cone production, though these are Pine, the only conifer which occurred in the not as marked as those of spruce, and pine- assumed ancestral habitat. eating crossbills also, on occasion, have to Recent findings are now casting doubt on move. For example, 1996 and 2001 were poor earlier assumptions. First, the main species of years for cones in Abernethy Forest, leading to crossbill inhabiting the remnants of the Cale- small numbers of crossbills there in these years. donian forest is not the Scottish Crossbill. The fact that colour-ringed birds reappeared Rather, it is mainly Parrot Crossbills which after these poor seasons, however, suggests that occur in Abernethy Forest and the Mar Lodge

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woods, and to a lesser extent in Glen Tanar tion with that species (Marquiss & Rae 2002). (Marquiss & Rae 2002; Summers 2002). Second, Alternatively, it is possible that the niche of the although Scottish Crossbills do occur in the Scottish Crossbill is somehow intermediate native pinewoods, they also occur in a range of between those of Parrot and Common Cross- conifer plantations, feeding on Lodgepole Pine, bills (as the bill size suggests), and that Scottish larch, and Sitka Spruce as well as Scots Pine Crossbills are able to utilise the range of planta- (Jardine 2002; Marquiss & Rae 2002; Summers tion woods now available in Scotland. Over the et al. 2002). Two possibilities exist to explain past 300 years, non-native conifers have been this situation. It is possible that the Scottish planted in the Highlands, so that there are now Crossbill was the main species occupying the three very common conifers: Sitka Spruce, ancient native pinewoods prior to the recent Lodgepole Pine, and the native Scots Pine invasions of Parrot Crossbills, but that it is now (Summers et al. 1999). There are also smaller excluded from these woods through competi- areas of Norway Spruce, larch (European Larch Ron W. Summers Ron W. Summers 68. Larch Larix sp. cones,The Aird, Inverness, 69. Lodgepole Pine Pinus contorta cone, Glen More, Highland, January 2003. Highland, April 2002. Ron W. Summers 71. A Scots Pine Pinus sylvestris cone, opening in Ron W. Summers spring, Abernethy Forest, Highland, May 2000. 70. Norway Spruce Picea abies cones, Strath Dearn, The seeds of Scots Pine are the main food of Highland, May 1996.The seeds from this conifer are Parrot Crossbills Loxia pytyopsittacus in the the main food of Common Crossbills Loxia Scottish Highlands, whereas Scottish Crossbills curvirostra in northern Europe. L. scotica feed from a range of conifers.

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Larix decidua, Japanese Larch L. kaempferi and a that species, and Scottish Crossbills were those hybrid L. × eurolepsis) and Douglas Fir Pseudot- within 0.2 mm of the mode for Scottish Cross- suga menziesii. It has been shown that birds can bill (Knox 1976; Piertney et al. 2001). Therefore, adapt quickly to changes in their environment. misclassification was unlikely. Genetic similarity It is even possible for the average bill size of a was assessed in two ways: by comparing the population to change significantly within a allele frequencies of five microsatellite loci, and single season if there is a strong selection pres- by sequence variation across the mitochondrial sure (Grant 1999). It is quite possible, therefore, control region. Both microsatellite and mito- that the bill size which characterises the Scottish chondrial DNA are considered neutral. In other Crossbill has evolved recently, a product of words, variation is not lost or maintained recent silviculture. A possible route is through through natural selection. Instead, variation hybridisation between Common and Parrot within a population can be accumulated by Crossbills (Marquiss & Rae 2002). mutation, and/or lost by random genetic drift. There is also indirect evidence of niche If two populations are kept isolated, they will overlap. If the three species of crossbill did tend to have very different frequencies of the occupy distinct niches, one would expect that genetic variants because mutation and drift act bill size would reflect these niches and would be independently in the two areas. Conversely, if equally differentiated, but this is not the case. two populations are hybridising, they will tend There is, in fact, a spread of bill sizes in Scotland to have very similar frequencies of the different covering the entire known range for crossbills genetic variants, because interbreeding will in Europe, and there is no clustering of bill sizes move DNA variants among the populations. into distinct groups (Marquiss & Rae 2002). Thus, examination of DNA markers such as microsatellite and mitochondrial DNA poly- Are Scottish Crossbills genetically distinct? morphisms is an ideal way of looking at the To examine the genetic relationships among degree of isolation between populations and Common, Scottish and Parrot Crossbills, non- species. invasive DNA samples were taken from cap- The key result of our findings was that the tured birds assigned to a putative crossbill type microsatellite and mitochondrial DNA of pop- based on bill size. Putative Parrot Crossbills ulations of the three crossbill types were not were those with bills greater than the modal bill statistically distinguishable (Piertney et al. depth for this species, Common Crossbills were 2001). The lack of differentiation could either those with bills less than the modal depth for be due to interbreeding among the three cross- Ron W. Summers 72. Scots Pine Pinus sylvestris plantation, Culbin Forest, Grampian, July 1999.

British Birds 96 • March 2003 • 100-111 107 The Scottish Crossbill – what we know and what we don’t bill types, or because there has been insufficient heterospecific pair (a male Parrot Crossbill was evolutionary time since separation from their found nesting with a female Scottish Crossbill common ancestral form for any differences to at Abernethy Forest), confirming that some accrue. The latter explanation is certainly pos- interbreeding does occur. sible, given that the Scottish Crossbill will have been separated from its ancestral form during Is the Scottish Crossbill a species? or after the last glaciation (10,000-20,000 years What does all this information tell us about the ago). From an evolutionary perspective, this is species status of the Scottish Crossbill? Defining an extremely short time for major genetic dif- a species is somewhat problematic, and has ferences to become apparent. That said, given sparked obdurate debate for several decades. that the crossbill population in Scotland would Several species definitions have been proposed have undergone a considerable reduction when and revised, though there is still little con- the Caledonian forest was reduced in size, evo- sensus. Four species concepts are commonly lutionary models would predict the Scottish applied to address species issues in avian taxa Crossbill to be more genetically distinct than we (Parkin in press). The data collected on the observed. Consequently, it is more likely that ecology, genetics, behaviour and morphology of some interbreeding does occur, and the chal- crossbills allow us to reach an informed opinion lenge is, therefore, to estimate how frequently about which of these concepts, if any, is most this takes place: whether the three species are appropriate, and whether they confirm or reject interbreeding freely where they occur sympatri- crossbills as distinct species. cally, or whether the genetic similarity among The Biological Species Concept (BSC; Mayr the forms is maintained with only occasional 1963) recognises that evolution leads to the interbreeding events. progressive genetic divergence between separate Although the genetic similarity of Common, populations such that ultimately they become Scottish and Parrot Crossbills might be taken to reproductively isolated. Thus, the BSC advo- indicate that the Scottish Crossbill cannot be a cates that species are ‘groups of interbreeding true species, it should be noted that we also natural populations that are reproductively iso- found no significant genetic differences lated from other such groups’. In the case of between Common and Parrot Crossbills crossbills, diagnostic call types may underpin (Piertney et al. 2001). These two types are behavioural isolating mechanisms preventing recognised as full species by most/all authorities interbreeding, and Knox (1990a) originally and show no overlap in bill measurements else- highlighted that the forms (Scottish and where in Europe. DNA studies to see whether Common Crossbills) do not interbreed when there are genetic differences between Common living sympatrically. These data would suggest and Parrot Crossbills on the European main- that crossbills satisfy species status under the land would help to clarify this puzzle. BSC, and, indeed, the Scottish Crossbill was ele- vated to species status because it was deemed to Do the three crossbill species interbreed? satisfy BSC criteria. Our more recent evidence We are now examining the extent of inter- of interbreeding from genetic and behavioural breeding of crossbills in Scotland by looking at data would, however, suggest that crossbill the mating patterns of birds and, more specifi- species are not reproductively isolated, so do not cally, the occurrence of heterospecific versus strictly satisfy species status under the BSC. It homospecific pairs, identified by bill depth and should, of course, be pointed out that there are call type. In a sense, this is what Knox (1990a) many species pairs (e.g. Herring Gull Larus did in his study in Deeside, but the sample of argentatus and Lesser Black-backed Gull L. pairs he examined was small, no details were fuscus) which exhibit occasional interbreeding, presented for the captured birds, and tape yet are still regarded as full species. Much recordings were not made of the pairs. Such depends on the frequency of interbreeding and details are required to be sure that the birds are the fertility of the hybrid offspring. actually a pair, and to confirm putative species. The Recognition Species Concept (RSC; It is likely that this survey will take several years Paterson 1985) claims that a species is ‘the most to generate a sufficiently large sample size from inclusive population of individual, biparental different forest types for useful conclusions to organisms which share a common fertilisation be drawn, but already we have evidence of one system’. In essence, individuals are defined as

108 British Birds 96 • March 2003 • 100-111 The Scottish Crossbill – what we know and what we don’t Ron W. Summers 73. Mixtures of planted conifers, now typical of Highland landscapes, East Ross-shire, March 1992. belonging to the same species if they share a to a Common Crossbill or a Parrot Crossbill as common mate-recognition system, be it behav- it is to another Scottish Crossbill (Piertney et al. ioural (such as song) or chemical (such as 2001), and as a consequence, the Scottish Cross- scent). This is similar to the BSC, but shifts bill cannot be viewed as a separate species attention from isolating mechanisms such as under the PSC. barriers to gene exchange, to a positive function The Evolutionary Species Concept (ESC; in facilitating reproduction only among indi- Simpson 1951) is the most general species vidual species members. As with the BSC, the concept, identifying a species as ‘an entity com- different, diagnostic calls of the crossbill types posed of organisms that maintains its identity may indicate species status under the RSC. from other such entities through time and over Given that we know mixed pairs of birds can space, and which has its own independent evo- occur, however, we do not know whether these lutionary fate and historical tendencies’. The calls are sufficient to reduce heterospecific maintenance of an intermediate bill depth matings, or whether such calls are plastic and would indicate that the Scottish Crossbill satis- can be learnt, as suggested by Groth (1993) for fies ESC criteria, though it is as yet unclear to captive-bred birds. what extent bill morphology is inherited, or The Phylogenetic Species Concept (PSC; how bill depth is affected by interbreeding. Cracraft 1983) defines a species as ‘the smallest Helbig et al. (2002) have attempted to syn- diagnosable cluster of individual organisms that thesize the species concepts to provide practical can be traced back to a single ancestral form, guidelines for assigning species rank with and within which there is a parental pattern of regard to the British List. They define species as ancestry and descent’.An outcome of this defin- population lineages maintaining their integrity ition is that species members are all more with respect to other such lineages through closely related to each other than they are to space and time, which means that species must anything else. The PSC avoids reference to iso- be diagnosably different (to recognise the sepa- lating mechanisms, and focuses instead on the rate lineages) and reproductively isolated (to phylogenetic histories of populations. In the maintain their integrity), and that their case of crossbills, genetic similarity among the members must have a common mate-recogni- forms (irrespective of whether that is caused by tion and fertilisation system (so that they can interbreeding or recent ancestry) means that a reproduce). The key is that a given species can Scottish Crossbill is as closely related genetically only be delimited in relation to other taxa, so

British Birds 96 • March 2003 • 100-111 109 The Scottish Crossbill – what we know and what we don’t must be diagnosable and, in the case of sym- evolutionary threshold of speciation, of which patric species, hybridise at such low frequency we are uncovering an increasing number from a that their gene pools will never merge. For range of different taxa (Orr & Smith 1998). crossbills in Scotland, this does not appear to be It is an inescapable truth that an endemic the case. The Scottish Crossbill cannot be diag- type of crossbill exists in Scotland which shows nosed based on morphology (bill size or body some specific adaptive divergence. This alone size) or genetic characteristics, although should warrant our best efforts to understand perhaps it may be identified by its call. More- its ecological requirements and to maintain it as over, as it can be argued that the gene pools of a component of our biodiversity. The important extant crossbills in Scotland are merged, it is scientific issues are therefore associated with difficult to be sure if they were ever truly sepa- what we need to do to maintain the Scottish rated. Crossbill. What is the population size? What A major criticism of all the species concepts woodland management and forestry policies is that they are attempting to partition a contin- and practices will maintain a population? How uous process (evolution) into artificially dis- frequent is interbreeding by sympatric crossbills crete entities (species). This is inherently and does the planting of exotic conifers difficult, and means that judgement is required promote it? What are the relative contributions in borderline cases. Recently, there have been of heritability and phenotypic plasticity in calls for a more rational basis for prioritising maintaining bill morphology? To what extent taxa of conservation concern, one which is are flight and excitement calls plastic and divorced from traditional taxonomy. Concepts learnt? Efforts to understand such issues should such as the Evolutionary Significant Unit (ESU) not be hampered just because the Scottish have been developed, and several operational Crossbill does not fall neatly into one of the definitions proposed, which identify genetic currently accepted species concepts. resources that warrant protection and allow a framework within which conservationists can Acknowledgments work. Waples (1991) defined an ESU as a The drafts were commented upon by C.W. Benkman, R. J. G. Dawson, P.Edelaar, D.W. Gibbons, R. E. Green, ‘…population that is substantially reproduc- D. C. Jardine, M. Marquiss and J.Wilson. tively isolated from other conspecific popula- tion units’, and which in a general sense References ‘represents an important component in the evo- Anon. 1995. Biodiversity: the UK Steering Group Report,Vol. lutionary legacy of the species’. Scottish cross- 2: Action Plans. HMSO, London. bills are clearly an extremely important Benkman, C.W. 1987. Food profitability and the foraging ecology of crossbills. Ecol. Monogr. 57: 251-267. component of the evolutionary legacy of cross- BOURC. 1980. Records committee: tenth report. Ibis 122: bills in the UK, Europe and the Palearctic. The 564-568. future conservation of crossbills depends on us Cracraft, J. 1983. Species concepts and speciation analysis. Current Ornithology 1: 159-187. taking on board the ESU concept in its broadest Gibbons, D.W., Reid, J. B., & Chapman, R. A. 1993. The New sense because this complex group of forms does Atlas of Breeding Birds in Britain and Ireland: 1988-1991. not fall clearly within any of the above species Poyser, London. concepts (Parkin in press). Grant, P.R. 1999. Ecology and Evolution of Darwin’s Finches. Princetown University Press, Princetown. Traditionally and pragmatically, species Gregory, R. D.,Wilkinson, N. I., Noble, D. G., Robinson, J. A., status is used by wildlife legislators as the basis Brown, A. F., Hughes, J., Procter, D., Gibbons D.W., & for allocating conservation priorities and is Galbraith, C. A. 2002.The population status of birds in the United Kingdom, Channel Islands and Isle of Man: used to assist designation of protected areas for an analysis of conservation concern 2002-2007. Brit. endangered animals and plants. Variation at Birds 95: 410-448. other levels is, however, an equally, if not more, Groth, J. G. 1993. Evolutionary differentiation in morphology, vocalizations, and allozymes among important component of biodiversity. More- nomadic sibling species in the North American Red over, as we learn more about the process of evo- Crossbill (Loxia curvirostra) complex. University Calif. lution, we appreciate that scenarios exist Publications Zool. 127: 1-143. whereby reproductive isolation, adaptive diver- Hartert, E. 1904. Die Vögel der Paläarktischen Fauna. Friedlander, Berlin. gence and neutral DNA differences all evolve Helbig, A. J., Knox, A. G., Parkin, D.T., Sangster, G., & and accrue at different rates. Conservation leg- Collinson, M. 2002. Guidelines for assigning species islation and priority setting which focus at the rank. Ibis 144: 518-525. Jardine, D. C. 1992. Crossbills in Scotland 1990 – an species level disadvantage those groups at the invasion year. Scottish Bird Report 23: 65-69.

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— 2002. Feeding rates of Scottish Crossbills on Sitka Microsatellite and mitochondrial DNA homogeneity Spruce. Scottish Birds 22: 108-109. among phenotypically diverse crossbill taxa in the UK. Knox, A. G. 1975. Crossbill taxonomy. In: Nethersole- Proc. Roy. Soc. Lond. B 268: 1511-1517. Thompson, D. Pine Crossbills. Poyser, Berkhamsted. Simpson, G. G. 1951.The species concept. Evolution 5: — 1976.The taxonomic status of the Scottish Crossbill 285-298. Loxia sp. Bull. B.O.C. 96: 15-19. Smout,T. C. 1993.Woodland history before 1850. In: — 1990a.The sympatric breeding of Common and Smout,T. C. (ed.), Scotland since Prehistory. Scottish Scottish Crossbills Loxia curvirostra and L. scotica and Cultural Press, Aberdeen. the evolution of crossbills. Ibis 132: 454-466. Summers, R.W. 1999. Numerical responses by crossbills — 1990b. Probable long-term sympatry of Common and Loxia spp. to annual fluctuations in cone crops. Ornis Scottish Crossbills in northeast Scotland. Scottish Birds Fennica 76: 141-144. 16: 11-18. — 2002. Parrot Crossbills breeding in Abernethy Forest, — 1990c. Identification of Crossbill and Scottish Crossbill. Highland. Brit. Birds 95: 4-11. Brit. Birds 83: 89-94. —, Jardine, D. C., Marquiss, M., & Rae, R. 2002.The Lack, D. 1944. Correlation between beak and food in the distribution and habitats of crossbills Loxia spp. in Crossbill Loxia curvirostra Linnaeus. Ibis 86: 552-553. Britain, with special reference to the Scottish Crossbill Marquiss, M., & Rae, R. 2002. Ecological differentiation in Loxia scotica. Ibis 144: 393-410. relation to bill size amongst sympatric, genetically —, Mavor, R. A., MacLennan, A. M., & Rebecca, G.W. 1999. undifferentiated crossbills Loxia spp. Ibis 144: 494-508. The structure of ancient native pinewoods and other —, —, Harvey, P., & Proctor, R. 1995. Scottish Crossbill woodlands in the Highlands of Scotland. Forest Ecology moves between Deeside and Strathspey. Scottish Bird and Management 119: 231-245. News 37: 2-3. Tipping, R. 1994. Form and fate of Scotland’s woodlands. Mayr, E. 1963. Populations, Species, and Evolution. Harvard Proceedings of the Society of Antiquaries of Scotland. 124: University Press, MA. 1-54. Murray, R. D. 1978. Crossbill evolution. Brit. Birds 71: 318- Thom,V. M. 1986. Birds in Scotland. Poyser, Calton. 319. Tucker, G. M., & Heath, M. F. 1994. Birds in Europe:Their Nethersole-Thompson, D. 1975. Pine Crossbills. Poyser, Conservation Status. BirdLife International, Cambridge. Berkhamsted. Voous, K. H. 1977. List of recent Holarctic bird species. Newton, I. 1972. Finches. Collins, London. Passerines. Ibis 119: 223-250, 376-406. Orr, M. R., & Smith,T. B. 1998. Ecology and speciation. — 1978.The Scottish Crossbill: Loxia scotica. Brit. Birds 71: Trends Ecol. Evol. 13: 502-506. 3-10. Parkin, D.T. In press. Birding and DNA: species for the Waples, R. S. 1991. Pacific salmon Oncorhynchus spp. and new millennium. Bird Study. the definition of ‘species’ under the Endangered Species Paterson, H. E. H. 1985.The recognition concept of Act. Marine Fisheries Rev. 53: 11-22. species. In:Vrba, E. S. (ed.), Species and Speciation. Witherby, H. F., Jourdain, F. C. R.,Ticehurst, N. F., & Tucker, Transvaal Museum Monograph No. 4.Transval Museum, B.W. 1943. The Handbook of British Birds,Vol. 1. Pretoria. Witherby, London. Piertney, S.B., Summers, R.W., & Marquiss, M. 2001.

Ron W. Summers, RSPB, Etive House, Beechwood Park, Inverness IV2 3BW Stuart B. Piertney, School of Biological Sciences, University of Aberdeen, Zoology Building, Tillydrone Avenue, Aberdeen AB24 2TZ

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