Red Crossbill (Loxia curvirostra) - HBW 15, p. 600

Taxonomy: Loxia curvirostra Linnaeus, 1758, Sweden.
Forms a species group with L. scotica and L. pytyopsittacus, and all have sometimes been considered conspecific. Taxonomy highly complicated and largely unresolved, requires much further study; recent genetic studies indicate limited differences between the three in Europe, indicating that they are not in total reproductive isolation, but extent of hybridization slight, and differences in bill size and in flight and excitement calls considered sufficient to restrict widespread interbreeding. Has apparently hybridized too with Carduelis pinus. In W Palearctic, geographical variation rather slight and clinal (nominate race becomes paler, brighter and larger from W to E, and within Europe brighter from N to S), and some races intergrade with others across wide areas, also considerable degree of individual variation; some races listed below possibly synonymous with more widespread ones, e.g. guillemardi perhaps better merged with nominate; further, several named races, e.g. hispana (described from near Murcia, in SE Spain), mariae (from Crimea), vasvarii (NW Turkey), caucasia (Caucasus) and ermaki (N Altai), proposed on basis of very slight differences or show intermediate characters; conversely, some other populations possibly merit subspecific recognition. Situation in E Palearctic and S Asia not yet investigated. In North America, numerous attempts made to classify populations of this species N of Mexican border into discrete races, with distinct and non-overlapping breeding ranges; described races reai (from Idaho) and vividior (from Colorado) may merit recognition, but present position complicated by identification of eight discrete types of flight call (across USA and W Canada) allied to other distinctive vocal characteristics (detectable through sonagrams) and morphological differences (mainly bill size or shape, and plumage colour) which may form basis for reproductive isolation of these taxa variously considered "cryptic species" or "pseudospecies"; studies reveal that crossbills appear to preserve their identity through specialization on particular species of conifer; in 2009, a newly discovered form from Idaho was proposed as a species, "L. sinesciuris" (see page 447), on basis of small morphological and vocal differences developed largely because of absence of competition for pine (Pinus) cones from squirrels (Sciuridae). Similar work in W Europe identified six vocal types, with some degree of assortative breeding and considerable overlap in area; in more recent study in W Mediterranean, however, a further six vocal types were described, thus placing some doubt on taxonomic value of these types. Further research required, especially as multiple vocal types now known to exist within crossbill populations, and adaptation to respective resource may be more appropriate explanation. Race pusilla has sometimes been listed as "percna". Nineteen subspecies currently recognized.