Spring Predictability and Bird Migration and Breeding Times: A Comment on the Phenomenon of Leap-Frog Migration

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Abstract
The time of the onset of spring, (i.e., of favorable weather conditions and of vegetational development) does not seem to be more predictable at high latitudes than at low (data analyzed from Norway), and thus cannot be used to explain the phenomenon of leap-frog migration in birds; i.e., it cannot explain why birds which breed in northern areas do not overwinter nearby as their more southern relatives do. When evaluating over-wintering and migration strategies in birds the basic points in comparison of different latitudinal areas are not only the phenological and climate predictability, but also the factors of competition and predation in the breeding areas, and the problem of building up enough resources to meet environmental stresses during the arrival and breeding.