Perhaps the most famous of irruptive migrants in the northern hemisphere are the crossbills, which feed year-round almost entirely on conifer seeds obtained directly from the cones (Newton 1972, 2006b). Three main boreal species are recognised: the Common (Red) Crossbill Loxia curvirostra and the Two-barred (White-winged) Crossbill L. leucoptera which occur in suitable coniferous habitat across North America and Eurasia, and the Parrot Crossbill L. pytyopsittacus which occurs in pine forests of northern Europe. Broadly speaking, these species differ in body and bill size, and specialise on different types of conifers, the Two-barred mainly on soft-coned species, the Common Crossbill mainly on medium-coned species, and the Parrot Crossbill on the very hard, thick-scaled cones of Scots Pine Pinus sylvestris. In addition, the Common Crossbill also varies in body and bill size from region to region across its extensive range, in association with the particular species of conifers that grow there (Box 18.3).

Box 18.3 The different types of Common (Red) Crossbills

In Europe, large-billed races of Common (Red) Crossbills Loxia curvirostra occur in the southern parts of the range, in the isolated mountain pine forests of: (1) southern Spain and the Balearic Islands (L. c. balearica); (2) southern Italy, Sicily and Northwest Africa (L. c. poliogyna); (3) the southern Balkans, Greece and Cyprus (L. c. guillemardi); (4) Corsica (L. c. corsicana); (5) the Crimea (L. c. mariae); and (6) Scotland, the latter regarded by some as a distinct species Loxia scotica. The first two of these large-billed subspecies (1 and 2) feed mainly on seeds of Aleppo Pine Pinus halapensis, the next two (3 and 4) mainly on Black Pine P. nigra, the Crimean Crossbill (5) mainly on Black Pine and Scots Pine P. sylvestris, and the Scottish Crossbill (6) mainly on Scots Pine.

In addition to these recognised types, Common Crossbills of the nominate subspecies, but with slightly different bill-types, have been represented in different invasions to western Europe, perhaps reflecting different boreal source areas (Davis 1964, Herremans 1988). The different types of crossbills also have call-notes which sound slightly different to the practised ear. In northwestern Europe alone, Robb (2000) described six different vocal types of Common Crossbills, as well as three other recognised 'species' (Two-barred, Parrot and Scottish). He showed that these calls were distinctive and constant, and suggested that they represent as yet undescribed 'cryptic species' (for other details, see Summers et al. 2002, Summers & Piertney 2003). Clearly, there may be more geographical variation in crossbills than at first meets the eye. Moreover, birds of different bill types tend to concentrate in localities with different conifer species, apparently reflecting different food plant preferences. In northern Scotland after an invasion, spatial segregation was evident even over distances as small as a few tens of kilometres, with large-billed crossbills mainly in pine areas and small-billed ones mainly in spruce and larch areas (Marquiss & Rae 1994, 2002, Summers et al. 1996).

Among Red Crossbills in North America, birds of eight different types are currently recognised, each type occupying a mainly different area from the others, and adapted in body size, bill size and palate structure to different conifer species (Benkman 1993, Groth 1988, 1991, 1993a, 1993b). The biggest type is almost twice the size of the smallest, and has a much larger bill. Usually all the birds in a flock are of the same type with their own distinct calls, voice again being a useful key to identification. Each type of crossbill apparently depends on at least one key conifer species, defined as one which normally produces cones somewhere in its wide range every year, holds its seeds in the cones at least until late winter, and for one reason or another does not get totally eaten by other seed-eaters (Benkman 1993). Each bill type appears adapted primarily to one species of conifer, even though all types are able to use several other conifers, albeit with lower rates of seed intake (as measured on both wild and captive birds).

The different types of crossbills may represent different species or at least different nomadic populations that are almost, if not completely, reproductively isolated from other types of crossbill (for occasional cases of interbreeding, see Benkman 1993, Adkisson 1996). Paired birds are almost always of the same morphological or call type, and juveniles develop flight calls like their parents (Groth 1993b). In some places after irruptions, birds of two or more call types may occasionally breed in the same place simultaneously, staying with their own kind and interbreeding rarely, if at all (Griscom 1937, Groth 1988, Knox 1992), or birds of different call types may use the same localities in different years from one another (Griscom 1937, Knox 1992). Genetic differences between the types are small and inconsistent with their calls, allozyme analyses suggesting that they diverged relatively recently, within the last 100 000 years (Groth 1993a). In addition, little genetic difference was evident between the Red Crossbill and White-winged Crossbill which, from study of their mitochrondrial DNA, were estimated to have diverged only about one million years ago (Groth 1991, 1993a, Questiau et al. 1999, Piertney et al. 2001).

White-winged Crossbills Loxia leucoptera, which have finer bills than most types of Red Crossbill, appear to show less geographical variation, but the North American race L. l. leucoptera has a smaller and narrower bill than the Eurasian L. l. bifasciata, and an even larger-billed, isolated pine-feeding form of the White-winged Crossbill (L. l. megaphaga) occurs on the island of Hispaniola, where it specialises on the seeds of the West Indian Pine Pinus occidentalis. Such geographical variations in crossbills imply that, despite their widespread wanderings, they are sufficiently faithful to particular regions to have become adapted to the conifers growing there.

Where several conifer species occur in the same region, crossbills switch from one species to another through the season, according to the different patterns of cone ripening and seed fall (Benkman 1987). Both Common and Two-barred Crossbills can breed in any month of the year, depending on the species of conifers available. However, most breeding occurs in late summer-autumn as cones mature, or in late winter-spring as cones open to release their seeds (Newton 1972, Benkman 1987, 1990).

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