Abstract
Leap-frog migration patterns are explicable in terms of the independent optimization of wintering location by different populations, without any need for assumptions concerning asymmetric competition between individuals from different breeding areas. The main factor influencing variation in choice of wintering site is the cost of spring migration in relation to its timing and the route taken. Among Fox Sparrows Passerella iliaca breeding on the west coast of Canada, later breeding northerly populations minimize the cost of spring migration by wintering in California, where food availability improves in early spring, enhancing conditions for premigratory fattening. This improvement occurs too late in the year for early breeding southerly populations which therefore truncate their migration. The southern wintering location of Alaskan populations is explicable if good conditions for premigratory fattening are particularly critical for populations undergoing a trans-oceanic migration.
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