Results/Conclusions Models based on the rate of intake and nitrogen (N) concentration of green grass provided the best model fits at both spatial scales tested, suggesting that digestive constraints and protein requirements may play key roles in driving migratory behaviour.  Tree cover and water availability improved model fits only marginally when added to forage intake and nutritional quality.  The emergence of a migration was dependent on the ability of the wildebeest to perceive changes in resource abundance at relatively large spatial scales (> 40-50 km) in the model.  When movement decisions were based solely on assessment of local conditions, the wildebeest failed to migrate across the ecosystem.  Our study highlights the potentially key role of strong and countervailing seasonally-driven rainfall and fertility gradients—a consistent feature of African savanna ecosystems—as drivers of long-distance seasonal migrations in ungulates.