Chapter 18 - Irruptive migrations: Boreal seed-eaters

Abstract

Irruptive migrants are those who show great flexibility in their movement patterns. Species known for their irruptive migrations are listed in this chapter, along with their main food plants. Most species eat mainly different foods in summer and winter and, like most birds, migrate twice each year, away from the breeding range in autumn and back in spring. Twice-yearly migrants generally move toward more southerly latitudes in autumn and to more northerly ones in spring, but they may concentrate to breed and winter in different areas in different years, wherever their food is plentiful at the time. In at least some irruptive migrants, the timing of autumn migration appears much more variable than among regular migrants, being markedly earlier in the big years. Correlations between numbers of birds and size of fruit crops in wintering localities have been documented in several species of irruptive migrants, but mostly in northern parts of the wintering range. As in many other migrants, the females of irruptive species often move earlier, in greater proportion or further than males, while juveniles of both sexes often move earlier, in greater proportion and further than adults. It is not only the autumn distances that vary from year to year in irruptive seed-eaters. The same holds for the spring distances, with many individuals settling in different areas in different years, depending on food supplies. The chapter explains irruptive migrations, the causes, and effects with the help of examples.