Summary

Related species with similar ecology often occupy mainly different geographical ranges and, in particular, many species which breed in the same region as one another segregate through migration to occupy different wintering ranges. This raises the possibility that, within the constraints of climate and food supplies, competition between species influences their migrations and wintering ranges. Similarly, within species, the distribution of each population may be related to the distribution of other populations, the east-west pattern in breeding areas often being maintained in wintering areas by parallel migrations. Hence, some longitudinal segregation of populations is more or less maintained year-round. An alternative pattern is fan migration in which birds from a large part of the breeding range may funnel into the narrow width of a small wintering range, or vice versa.

Three main latitudinal patterns have been described: (1) chain migration (northernmost breeding population wintering furthest north and southernmost furthest south); (2) leapfrog migration (northernmost breeding population wintering furthest south); or (3) telescopic migration (birds from a wide latitudinal span of breeding range concentrating in the same narrow latitudinal span of wintering range, or vice versa). Such patterns are apparent between closely related species and between different populations of the same species. Within species, social interactions could partly explain the migration differences between sex and age groups that give rise to latitudinal gradients in sex and age ratios across the wintering range.

These patterns are supposed to have resulted partly from selection pressure to reduce competition between individuals of different populations and species. However, non-breeding distributions have been examined, not just in terms of competition theory, but in terms of energy budgets, including the costs of wintering at different latitudes, and the costs of migrating different distances. They suggest that, while in general the daily energy needs for winter survival decline with decreasing latitude, as expected from the warmer temperatures, the energy needs for migration do not necessarily correlate well with the distance travelled, but also depend on the route taken and the prevailing winds. Nothing in the consideration of energy budgets is at odds with the idea of competition as a factor influencing migratory bird distributions.

Part Five

Migration Systems and Population

Limitation

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Sand Martins Riparia riparia, whose European breeding numbers are strongly affected by conditions in African wintering areas

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