Conclusions on twiceyearly migrants

In conclusion, the most striking feature of irruptive seed-eaters, compared with regular migrants, is the flexibility of their movements. In both breeding and wintering areas, irruptive migrants typically show huge numerical fluctuations from year to year, and much less site-fidelity than regular migrants, as individuals may breed or winter in widely separated areas in different years. In most species, the ringing records from different years mostly lie at different points on the same migration axis (northeast-southwest in much of Europe, northwest-southeast in much of North America), but some individuals show marked east or west displacement off this axis in different years. Typical irruptive migrants also show much more variation in their autumn migration dates and directions from year to year than do regular migrants, and make more frequent long movements within a winter. All these features are linked with the fluctuating and sporadic nature of the seed and fruit crops that provide their food. They contrast with the marked directional tendencies and strong homing and site-fidelity shown by birds that feed on more predictable food supplies, and that benefit from returning to the same familiar areas each year (Chapter 17).

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