The seed crops of some northern tree species vary greatly in size from year to year, both in particular regions and over the two northern land masses. However, seed crops in different regions fluctuate independently of one another, so that good crops in some regions coincide with poor crops in others. Each year, the birds that depend on tree seeds tend to settle at greatest densities in areas with the largest crops and, in line with the variable fruiting patterns, some individuals breed and winter in widely separated areas in different years. In different breeding seasons, as in different winters, ringed individuals of several species have been found at localities hundreds or thousands of kilometres apart. In years of widespread food shortage (or high numbers relative to food supplies), extending over many thousands or millions of square kilometres, large numbers of birds migrate to lower latitudes, as an irruptive migration. The timing of autumn migration, and the distances moved, vary greatly from year to year; in years of widespread crop failure birds often depart earlier and travel much further than usual. In North America, irruptions of several species occur about every second year, but in Europe irruptions are much less regular and less synchronised between species.

Crossbills depend year-round on conifer seeds. In much of Europe, Common Crossbills Loxia curvirostra make one major movement each year, in summer, between the shedding of one seed crop in certain areas and the formation of the next crop in other areas. In years of widespread crop failure, the movements become more directional, and birds leave the regular range in large numbers, occurring as irrupting flocks well outside their usual range, only to return in a later year. The same holds for Spotted Nutcrackers Nucifraga caryocatactes in the boreal zone of Eurasia, while Clark's Nutcrackers N. columbiana in North America move from the western mountains to the neighbouring lowlands in years of crop failure.

Compared with regular (obligate) migrants, irruptive (facultative) migrants show much greater year-to-year variations in the proportions of individuals that migrate, and greater individual and year-to-year variations in the autumn timing, directions and distances of movements. The control systems are flexible in irrup-tive migrants, enabling individuals to respond to feeding conditions at the time. Regular and irruptive migrants probably represent opposite extremes of a continuum of migratory behaviour found among birds, from narrow and consistent at one end to broad and flexible at the other. Both systems are adaptive, the one to conditions in which resource levels are predictable temporally and regionally, and the other to conditions in which resource levels vary unpredictably. Depending on the predictability and stability of its food supply, the same species may behave as a resident or regular migrant in one part of its range, and as an irruptive migrant in another.

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Snowy Owl Nyctea scandiacus, a well-known irruptive migrant

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