In some years, apparently when high populations coincide with poor Norway Spruce crops over wide areas, Common Crossbills leave the boreal forest, and move southwestward through Europe, appearing in many places that lack suitable conifer habitat. For example, between 1880 and 2000, Common Crossbills irrupted into Britain on at least 40 occasions, at intervals of 1-9 years (Newton 1972, 2006b). Each time, the birds appeared sometime during late May-October, mainly in June-July. The fact that trapped migrants differed slightly in bill dimensions from one irruption to another (Davis 1964, Herremans 1988) suggests that not all movements originated from the same region (or that not all bill size categories were stimulated to move in the same years). Most of these irruptions included small numbers of other crossbill species (L. pytyopsittacus or L. leucoptera) (Newton 1972).

Irruptive movements occur at about the same time as normal annual movements, but are more directional (towards the southwest), and cover much longer distances (with extremes at more than 5000 km). These differences could be explained by the birds achieving a higher migratory state in irruption years. A similar change occurs in normal migrants as they switch in late summer from random dispersal movements to directional long-distance migration (for Barn Swallow Hirundo rustica see Ormerod 1991; for Willow Warbler Phylloscopus tro-chilus see Norman & Norman 1985).

Once on the move, Crossbills spread over a wide area, some reaching the extreme southwest of Europe. Thousands of irrupting Crossbills have now been ringed at trapping sites in central Europe, giving hundreds of recoveries (Newton 1972, 2006b, Payevsky 1971, Weber 1972, Schloss 1984). Almost all recoveries within the current cone year (June-May) were to the south and west (mostly southwest) of the ringing site, while many of those in later years were far to the northeast, in the boreal zone of northern Russia, almost all in a region west of the Urals between 57° and 62°N and 40° and 60°E (Figure 18.11; Newton 2006b). Overall, at least 29 adults caught on migration during different invasions through central Europe have now been recovered back in the boreal forest to the northeast, 1-3 years later. None was found in northern boreal forest within the same spruce year as it was ringed in central Europe (Newton 2006b). If the outward movement was stimulated by food shortage, birds would gain no advantage in returning to their area of origin before the next year's crop was ready. In addition to ring recoveries, observational evidence for eastward or northeastward return movements in the summers of non-invasion years is available from several localities (Newton 2006b). The return movement is much less conspicuous than the outward one, perhaps because it involves smaller numbers and occurs in more than one year. The main axis of migration is clearly northeast-southwest, as in most other seed-eaters, but unlike them, Crossbills remained for a year or more before returning.1

If irrupting Crossbills had remained in the regular range, they would almost certainly have starved, but by moving out, a proportion survived to return in a later year, and may even have bred in the interim. Hence, the adaptive value of periodic mass emigration is presumably the same as in regular annual migration, namely the avoidance of food shortage. Those irrupting birds that find areas of seeding conifers often remain to breed. Mostly they move away after one breeding season, but some irruptions have resulted in the longer-term colonisation of new areas (for example, conifer plantations in Britain).

1Gatter (1993) has suggested, on the basis of the flight directions of Common Crossbills Loxia curvirostra seen over an observation point in Germany, that the birds return from whence they came within a few months of arriving in an invasion year, in the autumn and winter. Ring recoveries give little support for this view, however, because almost all of the recoveries of birds ringed on irruptions (mostly in Germany) were to the south or west of ringing sites until May of the next year, while birds recovered in later years were more evenly distributed between southwest and northeast. In fact, all recoveries in the boreal forest of northern Russia came more than one year after ringing in central Europe (see above).

Figure 18.11 Recoveries of Common Crossbills Loxia curvirostra ringed at migration time in June-October of invasion years in Germany, and recovered in the following year (filled circles) and later years (open circles) respectively. All the 83 birds recovered in the following year (to 31 May) were in the invasion areas of western Europe, whereas 22 out of 44 birds recovered in later years were far to the northeast, in the boreal zone of northern Russia (significance of difference in distribution, x2 = 34.2, P < 0.001). Only movements more than 50 km are included. These recoveries indicate that some irrupting Crossbills return to their region of origin in a later year. Map compiled from the recoveries listed in Schloss (1984), which include recoveries from the invasions of 1930, 1935, 1953, 1956, 1959, 1962, 1963, 1972, 1977 and 1979, although two-thirds of all recoveries in Russia probably stem from the invasion of 1963. Schloss (1984) lists nine other recoveries in northern Russia of birds ringed in Germany in non-invasion years or outside the migration season, some of which are in Figure 18.12. In addition to the records shown, another Common Crossbill ringed at Falsterbo in Sweden on 12 August 1963 was recovered in a later year at Gayny in northern Russia at 60°18'N, 54°18'E (Roos 1984), and one ringed in Northamptonshire in England on 2 June 1991 was recovered 1958 km east-northeast on 14 April 1998 in Pskov in northern Russia (58°16'N, 28°54'E) (Clark et al. 2000). For other recoveries of birds ringed in Switzerland, see Newton (1972).

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