Observations and ring recoveries give some idea of the huge areas over which individual boreal seed-eaters may roam to find food supplies necessary for their survival and reproduction. With their ability to move rapidly over long distances, these species can exploit a specialist niche in a way that other, more sedentary, animals could not. But because of the sporadic nature of their food supplies, only parts of their vast geographical ranges may be occupied at one time. Individual movements of hundreds or thousands of kilometres have been recorded between the natal and breeding sites in different years, and between the breeding sites of successive years. Again, however, because the ring recoveries are supplied by members of the public, they carry the assumption that birds found in the breeding season were in fact nesting at the locality concerned. The movements of irrup-tive species clearly contrast with those of more regular migrants which exploit more stable food supplies and, in association, show much greater site-fidelity and smaller dispersal distances.
The advantage of strong, endogenous control of migration, as shown by regular obligate migrants, is that it can permit anticipatory behaviour, allowing birds to prepare for an event such as migration, before it becomes essential for survival, and facilitating fat deposition before food becomes scarce. But such a fixed control system is likely to be beneficial only in predictable circumstances, in which food supplies change in a consistent manner, and at about the same dates, from year to year. It is not suited to populations that have to cope with a large degree of spatial and temporal unpredictability in their food supplies. It is these aspects of food supply which probably result in irruptive migrants showing greater variations in autumn timing, directions and distances, selection having imposed less precision on these aspects than in regular migrants. Both regular and irregular systems are adaptive, but to different types of food supplies, the one to consistent and predictable, and the other to inconsistent and unpredictable (Table 18.5). Nevertheless, regular (obligate) and irruptive (facultative) migrants are best regarded, not as distinct categories, but as opposite ends of a continuum, with predominantly endogenous control (=rigidity) at one end and predominantly external control (=flexibility) at the other.
Many irruptive migrants vary geographically in their behaviour, being more strongly irruptive in some regions than in others. It is easy to appreciate how regional variation in the obligate/facultative balance might evolve within species, as food supplies across the breeding range change from the more predictable to the less predictable, partly in association with the diversity of food types available, and with the degree of their year-to-year fluctuation. This is little different in principle from the transition from resident to migratory found in many
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