Advances in the study of irruptive migration

This paper discusses the movement patterns of two groups of birds which are generally regarded as irruptive migrants, namely (a) boreal finches and others that depend on fluctuating tree-fruit crops, and (b) owls and others that depend on cyclically fluctuating rodent populations. Both groups specialise on food supplies which, in particular regions, fluctuate more than 100-fold from year to year. However, seed-crops in widely separated regions may fluctuate independently of one another, as may rodent populations, so that poor food supplies in one region may coincide with good supplies in another. If individuals are to have access to rich food supplies every year, they must often move hundreds or thousands of kilometres from one breeding area to another. In years of widespread food shortage (or high numbers relative to food supplies) extending many thousands or millions of square kilometres, large numbers of individuals migrate to lower latitudes, as an ‘irruptive migration’. For these reasons, the distribution of the population, in both summer and winter, varies greatly from year to year. In particular breeding areas, many species of irruptive migrants fluctuate in density according to food supplies at the time. The facts that the response to food change is rapid, and that increases in numbers from one year to the next are often greater than can be explained by high survival and reproduction from the previous year, imply that such year to year density changes are due mainly to movements. Ring recoveries and other data lend support to this view. In irruptive migrants, in contrast to regular migrants, site fidelity is poor, and few individuals return to the same breeding areas in successive years (apart from owls in the increase phase of a rodent cycle). Moreover, ring recoveries and radio-tracking confirm that the same individuals can breed in different years in areas separated by hundreds or thousands of kilometres. Extreme examples are provided by Common Crossbills Loxia curvirostra, in which individual adults were found in localities up to 3200 km apart in different breeding seasons, and Snowy Owls Nyctea scandiaca found in localities up to nearly 2000 km apart. The implication from irruptive migrations, that individuals can winter in widely separated localities in different years, is also supported by ring recoveries, at least in seed-eaters, in which individuals have been found in one winter hundreds or thousands of kilometres from where they were ringed in a previous winter. Most such shifts could be regarded as lying at different points on the same migration axis, but some were apparently on different axes, as the birds were recovered in winter far to the east or west of where they were ringed in a previous winter. Extreme examples include a Bohemian Waxwing Bombycilla garrulus (6000 km, Ukraine to Siberia), a Siskin Carduelis spinus (3000 km, Sweden to Iran), a Pine Siskin Carduelis pinus (3950 km, Quebec to California), and a Common Redpoll Carduelis flammea (8350 km, Belgium to China). Compared to 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 timing, directions and distances of movements. The control systems are flexible in irruptive migrants, enabling individuals to respond to feeding conditions at the time. However, regular and irruptive migrants are probably best regarded, not as distinct categories, but as representing 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 in space and time, and the other to conditions in which resource levels are unpredictable. 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, as exemplified by particular species of both seed-eaters and rodent-eaters.