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Introduction
There are three breeding species of crossbills in Scotland: the small-billed Common Crossbill Loxia curvirostra, which eats mainly the seeds of spruce Picea spp. trees, the large-billed Parrot Crossbill L. pytyopsittacus which eats Scots Pine Pinus sylvestris seeds, and the Scottish Crossbill L. scotica which is intermediate in size, and eats the seeds of several conifers (Gibbons et al. 1993, Cramp and Perrins 1994, Marquiss and Rae 2002, Summers et al. 2002). The Common Crossbill is the most numerous, especially during invasion years, after which some stay to breed before returning to the continent in a subsequent year (Newton 2006). The Parrot Crossbill is the least abundant (Gibbons et al. 1993). There have been sporadic records of this species for over a century (Millais 1886, Summers 2004), though the larger number of recent breeding records may have resulted from the invasions in the 1980s (Catley and Hursthouse 1985, Summers 2002).
The Scottish Crossbill is Britain's only endemic bird species and is a red-listed species of high conservation concern in the UK, although considered 'Data Deficient' under IUCN criteria due to a lack of detailed information about its population size and trends (BirdLife International 2004, Eaton et al. 2005). There is also limited knowledge of habitat use. Previously, the population was estimated (indeed little more than guessed) at 320 pairs and 1,500 adults in the 1970s (Nethersole-Thompson 1975: 186), and these estimates have prevailed in subsequent literature (Thom 1986, Batten et al. 1990). Nethersole-Thompson (1975) was probably the primary source for the estimates (reported as 300-1,250 pairs) in Birds in Europe (BirdLife International 2004) and subsequently changed to 900-3,900 individuals (www.birdlife.org/datazone).
Attempts to survey the distribution and numbers of Scottish Crossbills have hitherto been hampered by the lack of a method of field identification (Knox 1990, Gibbons et al. 1993). However, recent work has shown that there are distinct call-types amongst crossbills in Scotland associated with birds with different bill sizes, and hence species (Summers et al. 2002, 2007). This allowed mapping of the distribution of the different call-types (including the Scottish Crossbill) in Scotland (Summers et al. 2004). It also led to the development of a survey method using lures to attract crossbills for counting and...