Seasonality and time allocation as causes of leap-frog migration in the Yellow Wagtail Motacilla flava

Leap-frog migration is attributable to variation among populations in the cost of spring migration in relation to the optimal time for the start of breeding. Populations which breed in regions where spring arrives relatively late, and thus leave the wintering quarters later than conspecifics, are frequently able to take advantage of a surge in food availability in the most distant wintering sites in order to fatten quickly and arrive on their breeding grounds at the optimal time. If populations breeding in regions where spring arrives early were to use these wintering sites, they would have to fatten before the surge in food availability occurs, or else delay migration and arrive late on the breeding grounds, to the detriment of breeding success. The high cost of migration to distant sites for these early breeders outweighs the lower survival probability associated with more proximate wintering sites, so migration is truncated. Similarly a late spring surge in food availability south of the breeding range of a species may lead migrant northern breeding, populations to leap-frog resident southern populations. A model of the processes involved in leap-frog migration is derived and applied to the migration pattern of the Yellow Wagtail.