Because the cropping patterns of many northern tree species are more regular than random, they would be expected to impart some regularity to large-scale emigration.
Some evidence points to a cyclic rhythm in the irruptions of seed-eating species, notably the tendency to a biennial pattern in several species in North America. In addition, a seven-year rhythm emerged in the influxes of Two-barred Crossbills Loxia leucoptera in autumn to Finland during 1960-1988 (Larson & Tombre 1989), a 5-6 year rhythm in the irruptions of Pine Grosbeaks Pinicola enucleator to southeastern Canada during 1889-1936 (Speirs 1939), and a 4-5 year rhythm in the movements of Eurasian Jays Garrulus glandarius in the Swabian Alps of central Europe during 1954-1973 (Gatter 1974). Such regularity reflects patterns in tree-seed production, but may not necessarily persist over longer periods, and in many areas irruptions seem irregular.
However, invasions of irruptive seed-eaters have been studied chiefly in the reception areas, where the degree of regularity probably varies with distance from the breeding range. While every irruption might reach the nearest areas, only the largest would reach the furthest areas, as illustrated above for nutcrackers and others. Sometimes, invasions come not just in one year but in two or more successive years, hence the term echo flights. But these might involve birds from different parts of the breeding range, thus obscuring any regularity there might be in more local breeding populations. For example, most invasions of the Pine Grosbeak Pinicola enucleator into Germany came from the north, and the birds belonged to the European subspecies, but in 1892, a larger subspecies invaded from Siberia (Grote 1937). Similarly, Redpoll invasions in central Europe in 1985 and 1986 were mainly long-billed holboellii types and hence probably came from within the range of the Larch Larix dahaurica in Siberia (Two-barred and Red Crossbills also came from this area in 1985). Moreover, different Common Crossbill invasions to western Europe have involved birds with different bill sizes, as mentioned above, which might also indicate different areas of origin (Davis 1964, Herremans 1988).
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